Since this is a programming website, I wanted to explain how to calculate the public key hash for securing the public key. This is important because the algorithm is completely undocumented. (This is not even documented by the canonical RFC 7469 public key binding extension for HTTP !
tl; dr : Base64(SHA256(SubjectPublicKeyInfo))
A certificate is not a (simple) public key
The first important point to understand is that the public key is just a number (for example, 2048 bits for RSA). A certificate is a set of additional information (subject names, hashes, digital signatures).
It means that:
- public key is a number
- a certificate associates a website with this number (e.g. stackoverflow.com)
The public key output contains the hash of the public key. This is not a certificate hash. This allows you to renew certificates while maintaining your public key.
Certificate Structure
We need to extract the public key from the certificate, so we need to know its structure. The certificate structure is documented (awful) in the RFC 5280 profile - Internet X.509 Certificate of public key infrastructure and certificate revocation list (CRL)
In terms of programming, the certificate:
struct Certificate { TBSCertificate tbsCertificate, AlgorithmIdentifier signatureAlgorithm, BITSTRING signatureValue }
And this is the SubjectPublicKeyInfo inside tbsCertificate , which contains what we are looking for:
struct TBSCertificate { EXPLICIT version, CertificateSerialNumber serialNumber, AlgorithmIdentifier signature, Name issuer, Validity validity, Name subject, SubjectPublicKeyInfo subjectPublicKeyInfo,
We need to take SHA256 from the raw subjectPublicKeyInfo .
digest = SHA256(certificate.tbsCertificate.subjectPublicKeyInfo);
This base64 encoded value becomes the value that we enter in the response headers of our web server:
String pin = String.Format("max-age=%d; pin-sha256=\"%s\"", 31536000,
For instance:
Public-Key-Pins: max-age=31536000; pin-sha256="hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=";
But what am I actually a hash of?
As an example of computing the PKP SHA256 hash, I'm going to use the current Facebook.com certificate. You can go to Facebook, view the certificate and save it in DER PEM in base64 encoding:
Facebook.cer (encoded in PEM DER ASN.1)
-----BEGIN CERTIFICATE----- MIIH5DCCBsygAwIBAgIQDACZt9eJyfZmJjF+vOp8HDANBgkqhkiG9w0BAQsFADBw MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3 d3cuZGlnaWNlcnQuY29tMS8wLQYDVQQDEyZEaWdpQ2VydCBTSEEyIEhpZ2ggQXNz dXJhbmNlIFNlcnZlciBDQTAeFw0xNjEyMDkwMDAwMDBaFw0xODAxMjUxMjAwMDBa MGkxCzAJBgNVBAYTAlVTMRMwEQYDVQQIEwpDYWxpZm9ybmlhMRMwEQYDVQQHEwpN ZW5sbyBQYXJrMRcwFQYDVQQKEw5GYWNlYm9vaywgSW5jLjEXMBUGA1UEAwwOKi5m YWNlYm9vay5jb20wWTATBgcqhkjOPQIBBggqhkjOPQMBBwNCAASg8YyvpzmIaFsT Vg4VFbSnRe8bx+WFPCsE1GWKMTEi6qOS7WSdumWB47YSdtizC0Xx/wooFJxP3HOp s0ktoHbTo4IFSjCCBUYwHwYDVR0jBBgwFoAUUWj/kK8CB3U8zNllZGKiErhZcjsw HQYDVR0OBBYEFMuYKIyhcufiMqmaPfINoYFWoRqLMIHHBgNVHREEgb8wgbyCDiou ZmFjZWJvb2suY29tgg4qLmZhY2Vib29rLm5ldIIIKi5mYi5jb22CCyouZmJjZG4u bmV0ggsqLmZic2J4LmNvbYIQKi5tLmZhY2Vib29rLmNvbYIPKi5tZXNzZW5nZXIu Y29tgg4qLnh4LmZiY2RuLm5ldIIOKi54eS5mYmNkbi5uZXSCDioueHouZmJjZG4u bmV0ggxmYWNlYm9vay5jb22CBmZiLmNvbYINbWVzc2VuZ2VyLmNvbTAOBgNVHQ8B Af8EBAMCB4AwHQYDVR0lBBYwFAYIKwYBBQUHAwEGCCsGAQUFBwMCMHUGA1UdHwRu MGwwNKAyoDCGLmh0dHA6Ly9jcmwzLmRpZ2ljZXJ0LmNvbS9zaGEyLWhhLXNlcnZl ci1nNS5jcmwwNKAyoDCGLmh0dHA6Ly9jcmw0LmRpZ2ljZXJ0LmNvbS9zaGEyLWhh LXNlcnZlci1nNS5jcmwwTAYDVR0gBEUwQzA3BglghkgBhv1sAQEwKjAoBggrBgEF BQcCARYcaHR0cHM6Ly93d3cuZGlnaWNlcnQuY29tL0NQUzAIBgZngQwBAgIwgYMG CCsGAQUFBwEBBHcwdTAkBggrBgEFBQcwAYYYaHR0cDovL29jc3AuZGlnaWNlcnQu Y29tME0GCCsGAQUFBzAChkFodHRwOi8vY2FjZXJ0cy5kaWdpY2VydC5jb20vRGln aUNlcnRTSEEySGlnaEFzc3VyYW5jZVNlcnZlckNBLmNydDAMBgNVHRMBAf8EAjAA MIICsAYKKwYBBAHWeQIEAgSCAqAEggKcApoAdgCkuQmQtBhYFIe7E6LMZ3AKPDWY BPkb37jjd80OyA3cEAAAAVjl02IEAAAEAwBHMEUCIQDvWFsUeqWE/xwIYcXPvbb5 ExzfHBZTNwfnUf4RPO/lBgIgdOGmr0j7+u8/S+7tfFw71ZEjqpwJELl/sEFuQdPn pwQBLwCsO5rtf6lnR1cVnm19V1Zy+dmBAJQem97/7KExO3V4LQAAAVjl02IoAAAE AQEAYvnMV+BfP3Wrk4yFQE/Zx5WsjSabYOpLj1Tj5xFaoVoHdGqLCf/Hi+Vv0IRy ePKFBCSW0+3eA589+WnCDMwcJlBYeZV8MlvHFZg3a66Uhx/OAvoetb0mCtUpnmIE UwLX/eMNEvjg2qTH3/33ysCo2l25+/EcR8upF+2KIcmnk5WwaJzfq7cFPQc4Cvcz mTHasJi/jmVaIaJ9HC50g3dx584TQX26lDLddF/Li4uEbJ7TSopnTzjQdWBtWbMF h3bcfhFCKaqK2kIJV3bgup5HibEnZ2LPm6lekY072ZFCGM4QYc4ukqzou2JWCRmG o0dMHJhnvQXpnIQGwATqCD4Q1AB2AFYUBpov18Ls0/XhvUSyPsdGdrm8mRFcwO+U mFXWidDdAAABWOXTYrkAAAQDAEcwRQIgGhXXbwUO5bD4Ts/Q0gqZwUS2vl/A4Hem k7ovxl82v9oCIQCbtkflDXbcunY4MAQCbKlnesPGc/nftA84xDhJpxFHWQB3AO5L vbd1zmC64UJpH6vhnmajD35fsHLYgwDEe4l6qP3LAAABWOXTZBEAAAQDAEgwRgIh AKubngQoa5Iak8eCOrffH7Xx3AP1NMb5pFw35nt2VSeRAiEA47Kq1UQcDXIEsV+W nuPd9LM5kpdeu0+TiHKtTLRQr0swDQYJKoZIhvcNAQELBQADggEBADrNSsoonbj1 YGjwy9t9wP9+kZBwrNMO2n5N5fQNhGawkEAX+lXlzgm3TqYlTNi6sCFbPBAErim3 aMVlWuOlctgnjtAdmdWZ4qEONrBLHPGgukDJ3Uen/EC/gwK6KdBCb4Ttp6MMPY1c hb/ciTLi3QUUU4h4OJWqUjvccBCDs/LydNjKWZZTxLJmxRSmfpyCU3uU2XHHMNlo 8UTIlqZsOtdqhg7/Q/cvMDHDkcI/tqelmg0MD2H9KpcmAvVkwgjn+BVpv5HELl+0 EP0UhYknI1B6LBecJuj7jI26eXZdX35CYkpI/SZA9KK+OYKHh6vCxKqnRZ9ZQUOj XnIWKQeV5Hg= -----END CERTIFICATE-----
All of the above is encoded in base64. Let's decrypt it into hex:
Facebook.cer (DER ASN.1 encoded)
30 82 07 e4 30 82 06 cc a0 03 02 01 02 02 10 0c 00 99 b7 d7 89 c9 f6 66 26 31 7e bc ea 7c 1c 30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00 30 70 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 15 30 13 06 03 55 04 0a 13 0c 44 69 67 69 43 65 72 74 20 49 6e 63 31 19 30 17 06 03 55 04 0b 13 10 77 77 77 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 31 2f 30 2d 06 03 55 04 03 13 26 44 69 67 69 43 65 72 74 20 53 48 41 32 20 48 69 67 68 20 41 73 73 75 72 61 6e 63 65 20 53 65 72 76 65 72 20 43 41 30 1e 17 0d 31 36 31 32 30 39 30 30 30 30 30 30 5a 17 0d 31 38 30 31 32 35 31 32 30 30 30 30 5a 30 69 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 13 30 11 06 03 55 04 08 13 0a 43 61 6c 69 66 6f 72 6e 69 61 31 13 30 11 06 03 55 04 07 13 0a 4d 65 6e 6c 6f 20 50 61 72 6b 31 17 30 15 06 03 55 04 0a 13 0e 46 61 63 65 62 6f 6f 6b 2c 20 49 6e 63 2e 31 17 30 15 06 03 55 04 03 0c 0e 2a 2e 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d 30 59 30 13 06 07 2a 86 48 ce 3d 02 01 06 08 2a 86 48 ce 3d 03 01 07 03 42 00 04 a0 f1 8c af a7 39 88 68 5b 13 56 0e 15 15 b4 a7 45 ef 1b c7 e5 85 3c 2b 04 d4 65 8a 31 31 22 ea a3 92 ed 64 9d ba 65 81 e3 b6 12 76 d8 b3 0b 45 f1 ff 0a 28 14 9c 4f dc 73 a9 b3 49 2d a0 76 d3 a3 82 05 4a 30 82 05 46 30 1f 06 03 55 1d 23 04 18 30 16 80 14 51 68 ff 90 af 02 07 75 3c cc d9 65 64 62 a2 12 b8 59 72 3b 30 1d 06 03 55 1d 0e 04 16 04 14 cb 98 28 8c a1 72 e7 e2 32 a9 9a 3d f2 0d a1 81 56 a1 1a 8b 30 81 c7 06 03 55 1d 11 04 81 bf 30 81 bc 82 0e 2a 2e 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d 82 0e 2a 2e 66 61 63 65 62 6f 6f 6b 2e 6e 65 74 82 08 2a 2e 66 62 2e 63 6f 6d 82 0b 2a 2e 66 62 63 64 6e 2e 6e 65 74 82 0b 2a 2e 66 62 73 62 78 2e 63 6f 6d 82 10 2a 2e 6d 2e 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d 82 0f 2a 2e 6d 65 73 73 65 6e 67 65 72 2e 63 6f 6d 82 0e 2a 2e 78 78 2e 66 62 63 64 6e 2e 6e 65 74 82 0e 2a 2e 78 79 2e 66 62 63 64 6e 2e 6e 65 74 82 0e 2a 2e 78 7a 2e 66 62 63 64 6e 2e 6e 65 74 82 0c 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d 82 06 66 62 2e 63 6f 6d 82 0d 6d 65 73 73 65 6e 67 65 72 2e 63 6f 6d 30 0e 06 03 55 1d 0f 01 01 ff 04 04 03 02 07 80 30 1d 06 03 55 1d 25 04 16 30 14 06 08 2b 06 01 05 05 07 03 01 06 08 2b 06 01 05 05 07 03 02 30 75 06 03 55 1d 1f 04 6e 30 6c 30 34 a0 32 a0 30 86 2e 68 74 74 70 3a 2f 2f 63 72 6c 33 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 2f 73 68 61 32 2d 68 61 2d 73 65 72 76 65 72 2d 67 35 2e 63 72 6c 30 34 a0 32 a0 30 86 2e 68 74 74 70 3a 2f 2f 63 72 6c 34 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 2f 73 68 61 32 2d 68 61 2d 73 65 72 76 65 72 2d 67 35 2e 63 72 6c 30 4c 06 03 55 1d 20 04 45 30 43 30 37 06 09 60 86 48 01 86 fd 6c 01 01 30 2a 30 28 06 08 2b 06 01 05 05 07 02 01 16 1c 68 74 74 70 73 3a 2f 2f 77 77 77 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 2f 43 50 53 30 08 06 06 67 81 0c 01 02 02 30 81 83 06 08 2b 06 01 05 05 07 01 01 04 77 30 75 30 24 06 08 2b 06 01 05 05 07 30 01 86 18 68 74 74 70 3a 2f 2f 6f 63 73 70 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 30 4d 06 08 2b 06 01 05 05 07 30 02 86 41 68 74 74 70 3a 2f 2f 63 61 63 65 72 74 73 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 2f 44 69 67 69 43 65 72 74 53 48 41 32 48 69 67 68 41 73 73 75 72 61 6e 63 65 53 65 72 76 65 72 43 41 2e 63 72 74 30 0c 06 03 55 1d 13 01 01 ff 04 02 30 00 30 82 02 b0 06 0a 2b 06 01 04 01 d6 79 02 04 02 04 82 02 a0 04 82 02 9c 02 9a 00 76 00 a4 b9 09 90 b4 18 58 14 87 bb 13 a2 cc 67 70 0a 3c 35 98 04 f9 1b df b8 e3 77 cd 0e c8 0d dc 10 00 00 01 58 e5 d3 62 04 00 00 04 03 00 47 30 45 02 21 00 ef 58 5b 14 7a a5 84 ff 1c 08 61 c5 cf bd b6 f9 13 1c df 1c 16 53 37 07 e7 51 fe 11 3c ef e5 06 02 20 74 e1 a6 af 48 fb fa ef 3f 4b ee ed 7c 5c 3b d5 91 23 aa 9c 09 10 b9 7f b0 41 6e 41 d3 e7 a7 04 01 2f 00 ac 3b 9a ed 7f a9 67 47 57 15 9e 6d 7d 57 56 72 f9 d9 81 00 94 1e 9b de ff ec a1 31 3b 75 78 2d 00 00 01 58 e5 d3 62 28 00 00 04 01 01 00 62 f9 cc 57 e0 5f 3f 75 ab 93 8c 85 40 4f d9 c7 95 ac 8d 26 9b 60 ea 4b 8f 54 e3 e7 11 5a a1 5a 07 74 6a 8b 09 ff c7 8b e5 6f d0 84 72 78 f2 85 04 24 96 d3 ed de 03 9f 3d f9 69 c2 0c cc 1c 26 50 58 79 95 7c 32 5b c7 15 98 37 6b ae 94 87 1f ce 02 fa 1e b5 bd 26 0a d5 29 9e 62 04 53 02 d7 fd e3 0d 12 f8 e0 da a4 c7 df fd f7 ca c0 a8 da 5d b9 fb f1 1c 47 cb a9 17 ed 8a 21 c9 a7 93 95 b0 68 9c df ab b7 05 3d 07 38 0a f7 33 99 31 da b0 98 bf 8e 65 5a 21 a2 7d 1c 2e 74 83 77 71 e7 ce 13 41 7d ba 94 32 dd 74 5f cb 8b 8b 84 6c 9e d3 4a 8a 67 4f 38 d0 75 60 6d 59 b3 05 87 76 dc 7e 11 42 29 aa 8a da 42 09 57 76 e0 ba 9e 47 89 b1 27 67 62 cf 9b a9 5e 91 8d 3b d9 91 42 18 ce 10 61 ce 2e 92 ac e8 bb 62 56 09 19 86 a3 47 4c 1c 98 67 bd 05 e9 9c 84 06 c0 04 ea 08 3e 10 d4 00 76 00 56 14 06 9a 2f d7 c2 ec d3 f5 e1 bd 44 b2 3e c7 46 76 b9 bc 99 11 5c c0 ef 94 98 55 d6 89 d0 dd 00 00 01 58 e5 d3 62 b9 00 00 04 03 00 47 30 45 02 20 1a 15 d7 6f 05 0e e5 b0 f8 4e cf d0 d2 0a 99 c1 44 b6 be 5f c0 e0 77 a6 93 ba 2f c6 5f 36 bf da 02 21 00 9b b6 47 e5 0d 76 dc ba 76 38 30 04 02 6c a9 67 7a c3 c6 73 f9 df b4 0f 38 c4 38 49 a7 11 47 59 00 77 00 ee 4b bd b7 75 ce 60 ba e1 42 69 1f ab e1 9e 66 a3 0f 7e 5f b0 72 d8 83 00 c4 7b 89 7a a8 fd cb 00 00 01 58 e5 d3 64 11 00 00 04 03 00 48 30 46 02 21 00 ab 9b 9e 04 28 6b 92 1a 93 c7 82 3a b7 df 1f b5 f1 dc 03 f5 34 c6 f9 a4 5c 37 e6 7b 76 55 27 91 02 21 00 e3 b2 aa d5 44 1c 0d 72 04 b1 5f 96 9e e3 dd f4 b3 39 92 97 5e bb 4f 93 88 72 ad 4c b4 50 af 4b 30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00 03 82 01 01 00 3a cd 4a ca 28 9d b8 f5 60 68 f0 cb db 7d c0 ff 7e 91 90 70 ac d3 0e da 7e 4d e5 f4 0d 84 66 b0 90 40 17 fa 55 e5 ce 09 b7 4e a6 25 4c d8 ba b0 21 5b 3c 10 04 ae 29 b7 68 c5 65 5a e3 a5 72 d8 27 8e d0 1d 99 d5 99 e2 a1 0e 36 b0 4b 1c f1 a0 ba 40 c9 dd 47 a7 fc 40 bf 83 02 ba 29 d0 42 6f 84 ed a7 a3 0c 3d 8d 5c 85 bf dc 89 32 e2 dd 05 14 53 88 78 38 95 aa 52 3b dc 70 10 83 b3 f2 f2 74 d8 ca 59 96 53 c4 b2 66 c5 14 a6 7e 9c 82 53 7b 94 d9 71 c7 30 d9 68 f1 44 c8 96 a6 6c 3a d7 6a 86 0e ff 43 f7 2f 30 31 c3 91 c2 3f b6 a7 a5 9a 0d 0c 0f 61 fd 2a 97 26 02 f5 64 c2 08 e7 f8 15 69 bf 91 c4 2e 5f b4 10 fd 14 85 89 27 23 50 7a 2c 17 9c 26 e8 fb 8c 8d ba 79 76 5d 5f 7e 42 62 4a 48 fd 26 40 f4 a2 be 39 82 87 87 ab c2 c4 aa a7 45 9f 59 41 43 a3 5e 72 16 29 07 95 e4 78
The binary code above is encoded in the ASN.1 DER version. We can use the excellent javascript ASN.1 decoder to decode it for us:
30 82 07 e4 ;30=SEQUENCE (0x07e4 bytes) |- tbsCertificate | 30 82 06 cc ;30=SEQUENCE (0x66cc bytes) | |- Version (0x03 bytes) | | a0 03 02 01 02 | |- SerialNumber (0x10 bytes) | | 02 10 0c 00 99 b7 d7 89 c9 f6 66 26 31 7e bc ea 7c 1c | |- Signature (0x0d bytes) | | 30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00 | |- Issuer (0x70 bytes) | | 30 70 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 15 30 13 06 03 55 04 0a 13 0c 44 69 67 69 43 65 72 74 20 49 6e 63 31 19 30 17 06 03 55 04 0b 13 10 77 77 77 2e 64 69 67 69 63 65 72 74 2e 63 6f 6d 31 2f 30 2d 06 03 55 04 03 13 26 44 69 67 69 43 65 72 74 20 53 48 41 32 20 48 69 67 68 20 41 73 73 75 72 61 6e 63 65 20 53 65 72 76 65 72 20 43 41 | |- Validity (0x1e bytes) | | 30 1e 17 0d 31 36 31 32 30 39 30 30 30 30 30 30 5a 17 0d 31 38 30 31 32 35 31 32 30 30 30 30 5a | |- Subject (0x69 bytes) | | 30 69 31 0b 30 09 06 03 55 04 06 13 02 55 53 31 13 30 11 06 03 55 04 08 13 0a 43 61 6c 69 66 6f 72 6e 69 61 31 13 30 11 06 03 55 04 07 13 0a 4d 65 6e 6c 6f 20 50 61 72 6b 31 17 30 15 06 03 55 04 0a 13 0e 46 61 63 65 62 6f 6f 6b 2c 20 49 6e 63 2e 31 17 30 15 06 03 55 04 03 0c 0e 2a 2e 66 61 63 65 62 6f 6f 6b 2e 63 6f 6d | |- SubjectPublicKeyInfo (0x59 bytes) | | 30 59 | | |- AlgorithmIdentifier (0x13 bytes) | | | 30 13 | | | 06 07 2a 86 48 ce 3d 02 01 ;Algorithm Object Identifier - 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type) | | | 06 08 2a 86 48 ce 3d 03 01 07 ;Algorithm Object Identiifer - 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve) | | |- SubjectPublicKey (0x42 bytes) | | 03 42 | | 00 04 a0 f1 8c af a7 39 88 68 5b 13 56 0e 15 15 b4 a7 45 ef | | 1b c7 e5 85 3c 2b 04 d4 65 8a 31 31 22 ea a3 92 ed 64 9d ba | | 65 81 e3 b6 12 76 d8 b3 0b 45 f1 ff 0a 28 14 9c 4f dc 73 a9 | | b3 49 2d a0 76 d3 | |- issuerUniqueID [0] | a3 82 05 4a | 30 82 05 46 30 1f 06 03 55 1d 23 04 18 30 16 80 14 51 68 ff 90 af 02 07 75 3c cc d9 65 64 62 a2 12 b8 59 72 3b 30 1d 06 | 4b 8f 54 e3 e7 11 5a a1 5a 07 74 6a 8b 09 ff c7 8b e5 6f d0 84 72 78 f2 85 04 24 96 d3 ed de 03 9f 3d f9 69 c2 0c cc 1c | 26 50 58 79 95 7c 32 5b c7 15 98 37 6b ae 94 87 1f ce 02 fa 1e b5 bd 26 0a d5 29 9e 62 04 53 02 d7 fd e3 0d 12 f8 e0 da | a4 c7 df fd f7 ca c0 a8 da 5d b9 fb f1 1c 47 cb a9 17 ed 8a 21 c9 a7 93 95 b0 68 9c df ab b7 05 3d 07 38 0a f7 33 99 31 | da b0 98 bf 8e 65 5a 21 a2 7d 1c 2e 74 83 77 71 e7 ce 13 41 7d ba 94 32 dd 74 5f cb 8b 8b 84 6c 9e d3 4a 8a 67 4f 38 d0 | 75 60 6d 59 b3 05 87 76 dc 7e 11 42 29 aa 8a da 42 09 57 76 e0 ba 9e 47 89 b1 27 67 62 cf 9b a9 5e 91 8d 3b d9 91 42 18 | ce 10 61 ce 2e 92 ac e8 bb 62 56 09 19 86 a3 47 4c 1c 98 67 bd 05 e9 9c 84 06 c0 04 ea 08 3e 10 d4 00 76 00 56 14 06 9a | 2f d7 c2 ec d3 f5 e1 bd 44 b2 3e c7 46 76 b9 bc 99 11 5c c0 ef 94 98 55 d6 89 d0 dd 00 00 01 58 e5 d3 62 b9 00 00 04 03 | 00 47 30 45 02 20 1a 15 d7 6f 05 0e e5 b0 f8 4e cf d0 d2 0a 99 c1 44 b6 be 5f c0 e0 77 a6 93 ba 2f c6 5f 36 bf da 02 21 | 00 9b b6 47 e5 0d 76 dc ba 76 38 30 04 02 6c a9 67 7a c3 c6 73 f9 df b4 0f 38 c4 38 49 a7 11 47 59 00 77 00 ee 4b bd b7 | 75 ce 60 ba e1 42 69 1f ab e1 9e 66 a3 0f 7e 5f b0 72 d8 83 00 c4 7b 89 7a a8 fd cb 00 00 01 58 e5 d3 64 11 00 00 04 03 | 00 48 30 46 02 21 00 ab 9b 9e 04 28 6b 92 1a 93 c7 82 3a b7 df 1f b5 f1 dc 03 f5 34 c6 f9 a4 5c 37 e6 7b 76 55 27 91 02 | 21 00 e3 b2 aa d5 44 1c 0d 72 04 b1 5f 96 9e e3 dd f4 b3 39 92 97 5e bb 4f 93 88 72 ad 4c b4 50 af 4b |- signatureAlgorithm | 30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00 |- signatureValue 03 82 01 01 00 3a cd 4a ca 28 9d b8 f5 60 68 f0 cb db 7d c0 ff 7e 91 90 70 ac d3 0e da 7e 4d e5 f4 0d 84 66 b0 90 40 17 fa 55 e5 ce 09 b7 4e a6 25 4c d8 ba b0 21 5b 3c 10 04 ae 29 b7 68 c5 65 5a e3 a5 72 d8 27 8e d0 1d 99 d5 99 e2 a1 0e 36 b0 4b 1c f1 a0 ba 40 c9 dd 47 a7 fc 40 bf 83 02 ba 29 d0 42 6f 84 ed a7 a3 0c 3d 8d 5c 85 bf dc 89 32 e2 dd 05 14 53 88 78 38 95 aa 52 3b dc 70 10 83 b3 f2 f2 74 d8 ca 59 96 53 c4 b2 66 c5 14 a6 7e 9c 82 53 7b 94 d9 71 c7 30 d9 68 f1 44 c8 96 a6 6c 3a d7 6a 86 0e ff 43 f7 2f 30 31 c3 91 c2 3f b6 a7 a5 9a 0d 0c 0f 61 fd 2a 97 26 02 f5 64 c2 08 e7 f8 15 69 bf 91 c4 2e 5f b4 10 fd 14 85 89 27 23 50 7a 2c 17 9c 26 e8 fb 8c 8d ba 79 76 5d 5f 7e 42 62 4a 48 fd 26 40 f4 a2 be 39 82 87 87 ab c2 c4 aa a7 45 9f 59 41 43 a3 5e 72 16 29 07 95 e4 78
Too much
But the only part that worries us is SubjectPublicKeyInfo :
SubjectPublicKeyInfo (0x59 bytes) | 30 59 | |- AlgorithmIdentifier (0x13 bytes) | | 30 13 | | 06 07 2a 86 48 ce 3d 02 01 ;Algorithm Object Identifier - 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type) | | 06 08 2a 86 48 ce 3d 03 01 07 ;Algorithm Object Identiifer - 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve) | |- SubjectPublicKey (0x42 bytes) | 03 42 | 00 04 a0 f1 8c af a7 39 88 68 5b 13 56 0e 15 15 b4 a7 45 ef | 1b c7 e5 85 3c 2b 04 d4 65 8a 31 31 22 ea a3 92 ed 64 9d ba | 65 81 e3 b6 12 76 d8 b3 0b 45 f1 ff 0a 28 14 9c 4f dc 73 a9 | b3 49 2d a0 76 d3
If you take only these bytes:
30 59 30 13 06 07 2a 86 48 ce 3d 02 01 06 08 2a 86 48 ce 3d 03 01 07 03 42 00 04 a0 f1 8c af a7 39 88 68 5b 13 56 0e 15 15 b4 a7 45 ef 1b c7 e5 85 3c 2b 04 d4 65 8a 31 31 22 ea a3 92 ed 64 9d ba 65 81 e3 b6 12 76 d8 b3 0b 45 f1 ff 0a 28 14 9c 4f dc 73 a9 b3 49 2d a0 76 d3
And running them through SHA256 , you will get:
- SHA256: 854206f3b721ef511941885904492ad952812e386eb2c530ee747cc32b98eeb6 (hex)
You then base64 encode this and you get:
- base64: hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY =
This is the hash that you add in the headers of the HTTP response:
Public-Key-Pins: max-age=31536000; pin-sha256="hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY=";
Cross check
Our calculated values correspond to the PKP hash that SSL Labs returns :
You can also insert the original PEM encoded certificate (base64) into https://certpins.appspot.com/pin and confirm that it returns the same PKP hash:
Public-Key-Pins: max-age = 31536000; pin-sha256 = "hUIG87ch71EZQYhZBEkq2VKBLjhussUw7nR8wyuY7rY =";
Why SubjectPublicKeyInfo?
A public key is a number (or in the case of RSA, a pair of numbers):
- exponent (e.g. 65,537)
- module (for example, one hundred and fifty-four million rectangular triangular rectangular)
Over the years, there are several ways to store these two numbers:
SHH :
[lenghPrefix]ssh-rsa[lengthPrefix][exponent][lengthPrefix][modulus]
Xml
<RSAKeyValue> <Modulus>ANxn+vSe8nIdRSy0gHkGoJQnUIIJ3WfOV7hsSk9An9LRafuZXYUMB6H5RxtWFm72f7nPKlg2N5kpqk+oEuhPx4IrnXIqnN5vwu4Sbc/w8rjE3XxcGsgXUams3wgiBJ0r1/lLCd6a61xRGtj4+Vae+Ps3mz/TdGUkDf80dVek9b9V</Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue> / w8rjE3XxcGsgXUams3wgiBJ0r1 / lLCd6a61xRGtj4 + Vae + Ps3mz / TdGUkDf80dVek9b9V </ Modulus> <RSAKeyValue> <Modulus>ANxn+vSe8nIdRSy0gHkGoJQnUIIJ3WfOV7hsSk9An9LRafuZXYUMB6H5RxtWFm72f7nPKlg2N5kpqk+oEuhPx4IrnXIqnN5vwu4Sbc/w8rjE3XxcGsgXUams3wgiBJ0r1/lLCd6a61xRGtj4+Vae+Ps3mz/TdGUkDf80dVek9b9V</Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue>
PKCS # 1
-----BEGIN RSA PUBLIC KEY----- MIGJAoGBANxn+vSe8nIdRSy0gHkGoJQnUIIJ3WfOV7hsSk9An9LRafuZXY UMB6H5RxtWFm72f7nPKlg2N5kpqk+oEuhPx4IrnXIqnN5vwu4Sbc/w8rjE 3XxcGsgXUams3wgiBJ0r1/lLCd6a61xRGtj4+Vae+Ps3mz/TdGUkDf80dV ek9b9VAgMBAAE= -----END RSA PUBLIC KEY-----
PKCS # 1 decided to use the ASN.1 DER encoding option of the following structure:
RSAPublicKey ::= SEQUENCE { modulus INTEGER, -- n publicExponent INTEGER -- e }
Later, when new public key encryption systems appeared, they "borrowed" the RSAPublicKey structure, but added a prefix with the algorithm identifier :
SubjectPublicKeyInfo ::= SEQUENCE { algorithm AlgorithmIdentifier, subjectPublicKey RSAPublicKey }
Where is the AlgorithID for PKCS # 1 RSAPublicKey: 1.2.840.113549.1.1.1 , where is it taken from:
- 1 - assigned ISO OID
- 1.2 - ISO member body
- 1.2.840 - USA
- 1.2.840.113549 - RSADSI
- 1.2.840.113549.1 - PKCS
- 1.2.840.113549.1.1 - PKCS-1
This format will be displayed in base64 encoded files as:
-----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDcZ/r0nvJyHUUstIB5BqCUJ1CC Cd1nzle4bEpPQJ/S0Wn7mV2FDAeh+UcbVhZu9n+5zypYNjeZKapPqBLoT8eCK51y Kpzeb8LuEm3P8PK4xN18XBrIF1GprN8IIgSdK9f5SwnemutcURrY+PlWnvj7N5s/ 03RlJA3/NHVXpPW/VQIDAQAB -----END PUBLIC KEY-----
This is SubjectPublicKeyInfo from which you get the sha256 hash.
Inside each certificate there is SubjectPublicKeyInfo ; even a Windows EFS certificate.
You can use OpenSSL to:
- extract the SubjectPublicKeyInfo block and save it as a
BEGIN PUBLIC KEY file - hash this
- base64 is
People should stop treating OpenSSL as a magic cryptobox. Too often, people simply enter esoteric openssl commands to do something, instead of understanding what they are doing. Even the RFC public key binding does not try to explain what it is, but simply tells you to go to the all-knowing openssl. OpenSSL is neither the beginning nor the end of encryption management.