RSA Public Key format

Question

Where can i find some documentation on the format of an RSA public key   An RSA public key formatted by OpenSSH      ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQB nAmOjTmezNUDKYvEeIRf2YnwM9 uUG1d0BYsc8 tRtx RGi7N2lUbp728MXGwdnL9od4cItzky zVdLZE2cycOa18xBK9cOWmcKS0A8FYBxEQWJ q9YVUgZbFKfYGaGQxsER A0w fX8ALuk78ktP31K69LcQgxIsl7rNzxsoOQKJ CIxOGMMxczYTiEoLvQhapFQMs3FL96didKr QbrfB1WT6s3838SEaXfgZvLef1YB2xmfhbT9OXFE3FXvh2UPBfN ffE7iiayQf 2XR 8j4N4bW30DiPtOQLGUrH1y5X rpNZNlWW2 jGIxqZtgWg7lTy3mXy5x836Sj 6L   The same public key formatted for use in Secure Shell  RFC 4716 - The Secure Shell  SSH  Public Key File Format    ---- BEGIN SSH2 PUBLIC KEY ---- AAAAB3NzaC1yc2EAAAABJQAAAQB nAmOjTmezNUDKYvEeIRf2YnwM9 uUG1d0BYs c8 tRtx RGi7N2lUbp728MXGwdnL9od4cItzky zVdLZE2cycOa18xBK9cOWmcKS 0A8FYBxEQWJ q9YVUgZbFKfYGaGQxsER A0w fX8ALuk78ktP31K69LcQgxIsl7r NzxsoOQKJ CIxOGMMxczYTiEoLvQhapFQMs3FL96didKr QbrfB1WT6s3838SEaX fgZvLef1YB2xmfhbT9OXFE3FXvh2UPBfN ffE7iiayQf 2XR 8j4N4bW30DiPtOQ LGUrH1y5X rpNZNlWW2 jGIxqZtgWg7lTy3mXy5x836Sj 6L ---- END SSH2 PUBLIC KEY ----   The same public key formatted as an RSA public key  note the five -  and no space    -----BEGIN RSA PUBLIC KEY----- MIIBCgKCAQEA xGZ wcz9ugFpP07Nspo6U17l0YhFiFpxxU4pTk3Lifz9R3zsIsu ERwta7 fWIfxOo208ett jhskiVodSEt3QBGh4XBipyWopKwZ93HHaDVZAALi 2A  xTBtWdEo7XGUujKDvC2 aZKukfjpOiUI8AhLAfjmlcD UZ1QPh0mHsglRNCmpCw mwSXA9VNmhz PiB Dml4WWnKW VHo2ujTXxq7 efMU4H2fny3Se3KYOsFPFGZ1TN QSYlFuShWrHPtiLmUdPoP6CV2mML1tk l7DIIqXrQhLUKDACeM5roMx0kLhUWB8P  0uj1CNlNN4JRZlC7xFfqiMbFRU9Z4N6YwIDAQAB -----END RSA PUBLIC KEY-----   The hex dump of the base-64 encoded data   00 00 00 07 73 73 68 2d 72 73 61 00 00 00 01 25 00 00 01 00 7f 9c 09 8e 8d 39 9e cc d5 03 29 8b c4 78 84 5f d9 89 f0 33 df ee 50 6d 5d d0  16 2c 73 cf ed 46 dc 7e 44 68 bb 37 69 54 6e 9e f6 f0 c5 c6 c1 d9 cb f6 87 78 70 8b 73 93 2f f3 55 d2 d9 13 67 32 70 e6 b5 f3 10 4a f5 c3  96 99 c2 92 d0 0f 05 60 1c 44 41 62 7f ab d6 15 52 06 5b 14 a7 d8 19  a1 90 c6 c1 11 f8 0d 30 fd f5 fc 00 bb a4 ef c9 2d 3f 7d 4a eb d2 dc  42 0c 48 b2 5e eb 37 3c 6c a0 e4 0a 27 f0 88 c4 e1 8c 33 17 33 61 38  84 a0 bb d0 85 aa 45 40 cb 37 14 bf 7a 76 27 4a af f4 1b ad f0 75 59  3e ac df cd fc 48 46 97 7e 06 6f 2d e7 f5 60 1d b1 99 f8 5b 4f d3 97  14 4d c5 5e f8 76 50 f0 5f 37 e7 df 13 b8 a2 6b 24 1f ff 65 d1 fb c8  f8 37 86 d6 df 40 e2 3e d3 90 2c 65 2b 1f 5c b9 5f fa e9 35 93 65 59  6d be 8c 62 31 a9 9b 60 5a 0e e5 4f 2d e6 5f 2e 71 f3 7e 92 8f fe 8b   i ve read that there are a couple of formats      Key data may be encoded in three general ways          Binary DER-encoded format  This is sometimes called ASN 1 BER-encoded   PEM or base64 format  This is the same data as the DER-encoded file but it is encoded in base64 with additional header and footer lines   XML format       If it s ASN 1  the hex certainly doesn t look like it   What s the format of an RSA public key     See also   RSA Key Formats Abstract Syntax Notation One Convert Base64 Public Key to XML Signature  Decode PEM formats

User · Accepted Answer

You can t just change the delimiters from ---- BEGIN SSH2 PUBLIC KEY ---- to -----BEGIN RSA PUBLIC KEY----- and expect that it will be sufficient to convert from one format to another  which is what you ve done in your example    This article has a good explanation about both formats   What you get in an RSA PUBLIC KEY is closer to the content of a PUBLIC KEY  but you need to offset the start of your ASN 1 structure to reflect the fact that PUBLIC KEY also has an indicator saying which type of key it is  see RFC 3447   You can see this using openssl asn1parse and -strparse 19  as described in this answer   EDIT  Following your edit  your can get the details of your RSA PUBLIC KEY structure using grep -v -- -----   tr -d   n    base64 -d   openssl asn1parse -inform DER       0 d 0  hl 4 l  266 cons  SEQUENCE               4 d 1  hl 4 l  257 prim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d 1  hl 2 l    3 prim  INTEGER            010001   To decode the SSH key format  you need to use the data format specification in RFC 4251 too  in conjunction with RFC 4253       The  ssh-rsa  key format has the following specific encoding         string     ssh-rsa        mpint     e       mpint     n    For example  at the beginning  you get 00 00 00 07 73 73 68 2d 72 73 61  The first four bytes  00 00 00 07  give you the length  The rest is the string itself  73 s  68 h      -  73 73 68 2d 72 73 61 ssh-rsa  followed by the exponent of length 1  00 00 00 01 25  and the modulus of length 256  00 00 01 00 7f

User · Answer

Reference Decoder of CRL CRT CSR NEW CSR PRIVATE KEY  PUBLIC KEY RSA RSA Public Key Parser  RSA Public Key   -----BEGIN RSA PUBLIC KEY----- -----END RSA PUBLIC KEY-----   Encrypted Private Key   -----BEGIN RSA PRIVATE KEY----- Proc-Type  4 ENCRYPTED -----END RSA PRIVATE KEY-----   CRL   -----BEGIN X509 CRL----- -----END X509 CRL-----   CRT   -----BEGIN CERTIFICATE----- -----END CERTIFICATE-----   CSR  -----BEGIN CERTIFICATE REQUEST----- -----END CERTIFICATE REQUEST-----   NEW CSR  -----BEGIN NEW CERTIFICATE REQUEST----- -----END NEW CERTIFICATE REQUEST-----   PEM  -----BEGIN RSA PRIVATE KEY----- -----END RSA PRIVATE KEY-----   PKCS7  -----BEGIN PKCS7----- -----END PKCS7-----   PRIVATE KEY  -----BEGIN PRIVATE KEY----- -----END PRIVATE KEY-----   DSA KEY  -----BEGIN DSA PRIVATE KEY----- -----END DSA PRIVATE KEY-----   Elliptic Curve  -----BEGIN EC PRIVATE KEY----- -----END EC PRIVATE KEY-----   PGP Private Key  -----BEGIN PGP PRIVATE KEY BLOCK----- -----END PGP PRIVATE KEY BLOCK-----   PGP Public Key  -----BEGIN PGP PUBLIC KEY BLOCK----- -----END PGP PUBLIC KEY BLOCK-----

User · Answer

Starting from the decoded base64 data of an OpenSSL rsa-ssh Key  i ve been able to guess a format    00 00 00 07  four byte length prefix  7 bytes  73 73 68 2d 72 73 61   ssh-rsa  00 00 00 01  four byte length prefix  1 byte  25  RSA Exponent  e   25 00 00 01 00  four byte length prefix  256 bytes  RSA Modulus  n     7f 9c 09 8e 8d 39 9e cc d5 03 29 8b c4 78 84 5f d9 89 f0 33 df ee 50 6d 5d d0 16 2c 73 cf ed 46  dc 7e 44 68 bb 37 69 54 6e 9e f6 f0 c5 c6 c1 d9  cb f6 87 78 70 8b 73 93 2f f3 55 d2 d9 13 67 32  70 e6 b5 f3 10 4a f5 c3 96 99 c2 92 d0 0f 05 60  1c 44 41 62 7f ab d6 15 52 06 5b 14 a7 d8 19 a1  90 c6 c1 11 f8 0d 30 fd f5 fc 00 bb a4 ef c9 2d  3f 7d 4a eb d2 dc 42 0c 48 b2 5e eb 37 3c 6c a0  e4 0a 27 f0 88 c4 e1 8c 33 17 33 61 38 84 a0 bb  d0 85 aa 45 40 cb 37 14 bf 7a 76 27 4a af f4 1b  ad f0 75 59 3e ac df cd fc 48 46 97 7e 06 6f 2d  e7 f5 60 1d b1 99 f8 5b 4f d3 97 14 4d c5 5e f8  76 50 f0 5f 37 e7 df 13 b8 a2 6b 24 1f ff 65 d1  fb c8 f8 37 86 d6 df 40 e2 3e d3 90 2c 65 2b 1f  5c b9 5f fa e9 35 93 65 59 6d be 8c 62 31 a9 9b  60 5a 0e e5 4f 2d e6 5f 2e 71 f3 7e 92 8f fe 8b    The closest validation of my theory i can find it from RFC 4253      The  ssh-rsa  key format has the following specific encoding     string     ssh-rsa    mpint     e   mpint     n       Here the  e  and  n  parameters form the signature key blob    But it doesn t explain the length prefixes     Taking the random RSA PUBLIC KEY i found  in the question   and decoding the base64 into hex   30 82 01 0a 02 82 01 01 00 fb 11 99 ff 07 33 f6 e8 05 a4 fd 3b 36 ca 68  e9 4d 7b 97 46 21 16 21 69 c7 15 38 a5 39 37 2e 27 f3 f5 1d f3 b0 8b 2e  11 1c 2d 6b bf 9f 58 87 f1 3a 8d b4 f1 eb 6d fe 38 6c 92 25 68 75 21 2d  dd 00 46 87 85 c1 8a 9c 96 a2 92 b0 67 dd c7 1d a0 d5 64 00 0b 8b fd 80  fb 14 c1 b5 67 44 a3 b5 c6 52 e8 ca 0e f0 b6 fd a6 4a ba 47 e3 a4 e8 94  23 c0 21 2c 07 e3 9a 57 03 fd 46 75 40 f8 74 98 7b 20 95 13 42 9a 90 b0  9b 04 97 03 d5 4d 9a 1c fe 3e 20 7e 0e 69 78 59 69 ca 5b f5 47 a3 6b a3  4d 7c 6a ef e7 9f 31 4e 07 d9 f9 f2 dd 27 b7 29 83 ac 14 f1 46 67 54 cd  41 26 25 16 e4 a1 5a b1 cf b6 22 e6 51 d3 e8 3f a0 95 da 63 0b d6 d9 3e  97 b0 c8 22 a5 eb 42 12 d4 28 30 02 78 ce 6b a0 cc 74 90 b8 54 58 1f 0f  fb 4b a3 d4 23 65 34 de 09 45 99 42 ef 11 5f aa 23 1b 15 15 3d 67 83 7a  63 02 03 01 00 01   From RFC3447 - Public-Key Cryptography Standards  PKCS   1  RSA Cryptography Specifications Version 2 1      A 1 1 RSA public key syntax      An RSA public key should be represented with the ASN 1 type RSAPublicKey     RSAPublicKey     SEQUENCE        modulus           INTEGER   -- n      publicExponent    INTEGER   -- e           The fields of type RSAPublicKey have the following meanings          modulus is the RSA modulus n    publicExponent is the RSA public exponent e       Using Microsoft s excellent  and the only real  ASN 1 documentation    30 82 01 0a        SEQUENCE  0x010A bytes  266 bytes     02 82 01 01     INTEGER   0x0101 bytes  257 bytes        00           leading zero because high-bit  but number is positive       fb 11 99 ff 07 33 f6 e8 05 a4 fd 3b 36 ca 68        e9 4d 7b 97 46 21 16 21 69 c7 15 38 a5 39 37 2e 27 f3 f5 1d f3 b0 8b 2e        11 1c 2d 6b bf 9f 58 87 f1 3a 8d b4 f1 eb 6d fe 38 6c 92 25 68 75 21 2d        dd 00 46 87 85 c1 8a 9c 96 a2 92 b0 67 dd c7 1d a0 d5 64 00 0b 8b fd 80        fb 14 c1 b5 67 44 a3 b5 c6 52 e8 ca 0e f0 b6 fd a6 4a ba 47 e3 a4 e8 94        23 c0 21 2c 07 e3 9a 57 03 fd 46 75 40 f8 74 98 7b 20 95 13 42 9a 90 b0        9b 04 97 03 d5 4d 9a 1c fe 3e 20 7e 0e 69 78 59 69 ca 5b f5 47 a3 6b a3        4d 7c 6a ef e7 9f 31 4e 07 d9 f9 f2 dd 27 b7 29 83 ac 14 f1 46 67 54 cd        41 26 25 16 e4 a1 5a b1 cf b6 22 e6 51 d3 e8 3f a0 95 da 63 0b d6 d9 3e        97 b0 c8 22 a5 eb 42 12 d4 28 30 02 78 ce 6b a0 cc 74 90 b8 54 58 1f 0f        fb 4b a3 d4 23 65 34 de 09 45 99 42 ef 11 5f aa 23 1b 15 15 3d 67 83 7a        63     02 03           INTEGER  3 bytes        01 00 01   giving the public key modulus and exponent    modulus   0xfb1199ff0733f6e805a4fd3b36ca68   837a63 exponent   65 537     Update  My expanded form of this answer in another question

[encryption] RSA Public Key format

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