title: convert openSSH rsa key to javax.crypto.Cipher compatible format
body: <p>Is there a way to programmatically convert the <a href="http://www.jcraft.com/jsch/" rel="nofollow noreferrer">Jsch</a> generated SSH RSA-keys to a format <a href="http://docs.oracle.com/javase/7/docs/api/javax/crypto/Cipher.html" rel="nofollow noreferrer">javax.crypto.Cipher</a> can use for encryption? I have mostly seen answers similar to <a href="https://stackoverflow.com/a/7473874/1020470">this</a>:</p>
  <pre><code>openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key_file -nocrypt &gt; pkcs8_key
  </code></pre>
  <p>But I don't have acces to openSSL or shell commands. BTW: I'm only using JDK6.</p>
  <p>Thanks to @erickson for his help I can convert the Public Key from RFC4716 to a Java public key using the exponent and the modulus as <code>BigInteger</code> types passed to a <code>KeyFactory</code> with the <code>RSAPublicKeySpec</code>. His solution is below.</p>
  <p>Now I am trying to convert the private key. Here's an example <code>privateJsch.key</code> generated using <code>writePrivateKey(str filename)</code>:</p>
  <pre><code>-----BEGIN RSA PRIVATE KEY-----
  MIICXAIBAAKBgQC0ouLgTjmKjHU6UjNSL8HyTIdFM1UdVpgU81paWKreN8L36YoT
  goZQHeyyUCCHmq3r3cKaySyu93mHBY0l76qSAIRZgE1IAFkBhNWBdlJ9UYA9HXm/
  MqTQHbpqz0EYGE9TsFHS8dn1/utsJxKSWZ4xPNYjfS4Ps6G84iRwfdrIbQIDAQAB
  AoGAKv3xnY1AqLcRV5Yk3NS9Blwsfc3f3iG0BJh+0q3zzPvcjYCp+kbAjOTyZuYn
  N98asd6P6KMk3WfNJtOtanAGWl46bmtzNsQtSr5rVQEgs2w8i2yJcwVAYf2Td4qX
  m3dH+roJA/CEFRSDat4sUfjOVmsYQXIBa0W2XTpp+7T1U4ECQQD1wSR6iTz7Bja0
  MPcizDbRTRQHALBf7E8j8YOLpN/IGSox9pT+ktjsI2vMaD+b3SM4s0FD8quBlppE
  o5FAguHxAkEAvCrCK7eZU3H+Ul1iw9Kd3WPHjDvQcdT5rEL+NSYEZyHgU7ipXEih
  UHvK47Bkte/PVIu3jBFBnMujA0XiT0gSPQJBAI3+8j/nChgU6AjHfhRaIJZgzeCZ
  8k8KcFPZWWOXeUHZ4HqL+lz5pmMSuFecKJy7cn1xfZVwIs62oR5l0CiRN1ECQCui
  CqaSi3ZjH6M/znA0PbEhuxsUn7BVv5OncUUnzKuRmnAviO5CVU3Rdum3dJMPydcE
  Ewri0YEnY2SV5vWVc80CQH43uBbshz7ju3DdVykHFrRElQB+f0YMK3Ad7eu+us0w
  dLrOOoXP0T60B/bMTo8rdMa6XU/0w/w8FsOqoxNY23U=
  -----END RSA PRIVATE KEY-----
  </code></pre>
  <p>using <code>openssl asn1parse -in privateJsch.key -out privateJsch.der</code> yeild:</p>
  <pre><code>$ openssl asn1parse -in privateJsch.key -out privateJsch.der
      0:d=0  hl=4 l= 604 cons: SEQUENCE
      4:d=1  hl=2 l=   1 prim: INTEGER           :00
      7:d=1  hl=3 l= 129 prim: INTEGER           :B4A2E2E04E398A8C753A5233522FC1F24C874533551D569814F35A5A58AADE37C2F7E98A138286501DECB25020879AADEBDDC29AC92CAEF77987058D25EFAA92008459804D4800590184D58176527D51803D1D79BF32A4D01DBA6ACF4118184F53B051D2F1D9F5FEEB6C271292599E313CD6237D2E0FB3A1BCE224707DDAC86D
    139:d=1  hl=2 l=   3 prim: INTEGER           :010001
    144:d=1  hl=3 l= 128 prim: INTEGER           :2AFDF19D8D40A8B711579624DCD4BD065C2C7DCDDFDE21B404987ED2ADF3CCFBDC8D80A9FA46C08CE4F266E62737DF1AB1DE8FE8A324DD67CD26D3AD6A70065A5E3A6E6B7336C42D4ABE6B550120B36C3C8B6C8973054061FD93778A979B7747FABA0903F0841514836ADE2C51F8CE566B184172016B45B65D3A69FBB4F55381
    275:d=1  hl=2 l=  65 prim: INTEGER           :F5C1247A893CFB0636B430F722CC36D14D140700B05FEC4F23F1838BA4DFC8192A31F694FE92D8EC236BCC683F9BDD2338B34143F2AB81969A44A3914082E1F1
    342:d=1  hl=2 l=  65 prim: INTEGER           :BC2AC22BB7995371FE525D62C3D29DDD63C78C3BD071D4F9AC42FE3526046721E053B8A95C48A1507BCAE3B064B5EFCF548BB78C11419CCBA30345E24F48123D
    409:d=1  hl=2 l=  65 prim: INTEGER           :8DFEF23FE70A1814E808C77E145A209660CDE099F24F0A7053D95963977941D9E07A8BFA5CF9A66312B8579C289CBB727D717D957022CEB6A11E65D028913751
    476:d=1  hl=2 l=  64 prim: INTEGER           :2BA20AA6928B76631FA33FCE70343DB121BB1B149FB055BF93A7714527CCAB919A702F88EE42554DD176E9B774930FC9D704130AE2D18127636495E6F59573CD
    542:d=1  hl=2 l=  64 prim: INTEGER           :7E37B816EC873EE3BB70DD57290716B44495007E7F460C2B701DEDEBBEBACD3074BACE3A85CFD13EB407F6CC4E8F2B74C6BA5D4FF4C3FC3C16C3AAA31358DB75
  </code></pre>
  <p>base64 decoding the key portion yields the following in <em>hex</em> broken out into ASN.1 PKCS#1 components (see RFC3447) for this example <em>only</em>, others will follow pattern with <em>different</em> numbers of bytes:</p>
  <pre><code>30:82:    x3082 == ASN.1 Sequence
  02:5C:    Key Length == 604 bytes
  02:01:    x02 == ASN.1 integer, Value Length == 1 byte
  00:       Version == 0
  02:81:81: x02 == ASN.1 integer, Modulus Length == 129 bytes
  00:B4:A2:E2:E0:4E:39:8A:8C:75:3A:52:33:52:2F:C1:
  F2:4C:87:45:33:55:1D:56:98:14:F3:5A:5A:58:AA:DE:
  37:C2:F7:E9:8A:13:82:86:50:1D:EC:B2:50:20:87:9A:
  AD:EB:DD:C2:9A:C9:2C:AE:F7:79:87:05:8D:25:EF:AA:
  92:00:84:59:80:4D:48:00:59:01:84:D5:81:76:52:7D:
  51:80:3D:1D:79:BF:32:A4:D0:1D:BA:6A:CF:41:18:18:
  4F:53:B0:51:D2:F1:D9:F5:FE:EB:6C:27:12:92:59:9E:
  31:3C:D6:23:7D:2E:0F:B3:A1:BC:E2:24:70:7D:DA:C8:
  6D:       Modulus
  02:03:    x02 == ASN.1 integer, Value Length == 3 bytes
  01:00:01: Public Exponent
  02:81:80: x02 == ASN.1 integer, Value Length == 128 bytes
  2A:FD:F1:9D:8D:40:A8:B7:11:57:96:24:DC:D4:BD:06:
  5C:2C:7D:CD:DF:DE:21:B4:04:98:7E:D2:AD:F3:CC:FB:
  DC:8D:80:A9:FA:46:C0:8C:E4:F2:66:E6:27:37:DF:1A:
  B1:DE:8F:E8:A3:24:DD:67:CD:26:D3:AD:6A:70:06:5A:
  5E:3A:6E:6B:73:36:C4:2D:4A:BE:6B:55:01:20:B3:6C:
  3C:8B:6C:89:73:05:40:61:FD:93:77:8A:97:9B:77:47:
  FA:BA:09:03:F0:84:15:14:83:6A:DE:2C:51:F8:CE:56:
  6B:18:41:72:01:6B:45:B6:5D:3A:69:FB:B4:F5:53:81:
            Private Exponent 
  02:41:    x02 == ASN.1 integer, Value Length == 65 bytes
  00:F5:C1:24:7A:89:3C:FB:06:36:B4:30:F7:22:CC:36:
  D1:4D:14:07:00:B0:5F:EC:4F:23:F1:83:8B:A4:DF:C8:
  19:2A:31:F6:94:FE:92:D8:EC:23:6B:CC:68:3F:9B:DD:
  23:38:B3:41:43:F2:AB:81:96:9A:44:A3:91:40:82:E1:
  F1:       Prime P
  02:41:    x02 == ASN.1 integer, Value Length == 65 bytes
  00:BC:2A:C2:2B:B7:99:53:71:FE:52:5D:62:C3:D2:9D:
  DD:63:C7:8C:3B:D0:71:D4:F9:AC:42:FE:35:26:04:67:
  21:E0:53:B8:A9:5C:48:A1:50:7B:CA:E3:B0:64:B5:EF:
  CF:54:8B:B7:8C:11:41:9C:CB:A3:03:45:E2:4F:48:12:
  3D:       Prime Q
  02:41:    x02 == ASN.1 integer, Value Length == 65 bytes
  00:8D:FE:F2:3F:E7:0A:18:14:E8:08:C7:7E:14:5A:20:
  96:60:CD:E0:99:F2:4F:0A:70:53:D9:59:63:97:79:41:
  D9:E0:7A:8B:FA:5C:F9:A6:63:12:B8:57:9C:28:9C:BB:
  72:7D:71:7D:95:70:22:CE:B6:A1:1E:65:D0:28:91:37:
  51:       Prime P Exponent
  02:40:    x02 == ASN.1 integer, Value Length == 64 bytes
  2B:A2:0A:A6:92:8B:76:63:1F:A3:3F:CE:70:34:3D:B1:
  21:BB:1B:14:9F:B0:55:BF:93:A7:71:45:27:CC:AB:91:
  9A:70:2F:88:EE:42:55:4D:D1:76:E9:B7:74:93:0F:C9:
  D7:04:13:0A:E2:D1:81:27:63:64:95:E6:F5:95:73:CD:
            Prime Q Exponent
  02:40:    x02 == ASN.1 integer, Value Length == 64 bytes
  7E:37:B8:16:EC:87:3E:E3:BB:70:DD:57:29:07:16:B4:
  44:95:00:7E:7F:46:0C:2B:70:1D:ED:EB:BE:BA:CD:30:
  74:BA:CE:3A:85:CF:D1:3E:B4:07:F6:CC:4E:8F:2B:74:
  C6:BA:5D:4F:F4:C3:FC:3C:16:C3:AA:A3:13:58:DB:75
            CRT Coefficient
  </code></pre>
  <p>Similar posts:</p>
  <ul>
  <li><a href="https://stackoverflow.com/questions/3299003/how-to-convert-openssh-public-key-file-format-to-pem">how-to-convert-openssh-public-key-file-format-to-pem</a></li>
  <li><a href="https://stackoverflow.com/questions/3706177/how-to-generate-ssh-compatible-id-rsa-pub-from-java">how-to-generate-ssh-compatible-id-rsa-pub-from-java</a></li>
  <li><a href="https://stackoverflow.com/questions/3243018/how-to-load-rsa-private-key-from-file">how-to-load-rsa-private-key-from-file</a></li>
  <li><a href="https://stackoverflow.com/questions/3660132/formatting-rsa-keys-for-openssl-in-java">formatting-rsa-keys-for-openssl-in-java</a></li>
  <li>and many more, also on <a href="https://security.stackexchange.com">https://security.stackexchange.com</a></li>
  </ul>
  <p>references:</p>
  <ul>
  <li><a href="http://www.bouncycastle.org/java.html" rel="nofollow noreferrer">bouncycastle/java</a></li>
  <li><a href="https://polarssl.org/kb/cryptography/asn1-key-structures-in-der-and-pem" rel="nofollow noreferrer">asn1-key-structures-in-der-and-pem</a></li>
  <li><a href="https://www.rfc-editor.org/rfc/rfc3447" rel="nofollow noreferrer">RFC3447</a></li>
  <li><a href="https://www.rfc-editor.org/rfc/rfc4716" rel="nofollow noreferrer">RFC4716</a></li>
  <li><a href="http://etherhack.co.uk/asymmetric/docs/rsa_key_breakdown.html" rel="nofollow noreferrer">rsa_key_breakdown</a></li>
  </ul>