I do not find any tool or solutions in java that can do it. So, I read RFC 4253 for public key format description and RFC 4251 for mpint description.
Important parts from RFC 4253. section 6.6
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.
And important part from RFC 4251
mpint
Represents multiple precision integers in two's complement format,
stored as a string, 8 bits per byte, MSB first. Negative numbers
have the value 1 as the most significant bit of the first byte of
the data partition. If the most significant bit would be set for
a positive number, the number MUST be preceded by a zero byte.
Unnecessary leading bytes with the value 0 or 255 MUST NOT be
included. The value zero MUST be stored as a string with zero
bytes of data.
By convention, a number that is used in modular computations in
Z_n SHOULD be represented in the range 0 <= x < n.
Examples:
value (hex) representation (hex)
----------- --------------------
0 00 00 00 00
9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7
80 00 00 00 02 00 80
-1234 00 00 00 02 ed cc
-deadbeef 00 00 00 05 ff 21 52 41 11
Know all this, write code to convert ssh-rsa key to java format is easy.
private static int SIZEOF_INT = 4;
private static String key1 = "AAAAB3NzaC1yc2EAAAADAQABAAABAQClAxT5S/WuX04OXBt9R59WcL45OmaU3M5U063lfyja7ovqaVR7/2kHtLF/LoCQCXSZMny8RTCGDjoXD7G/tGsyHFDHCI//Y1VDLE06AlDzrlu69DQY91+6gkhGjH3SF6us5hXlihrbSFLfAlSdkEs8gwSrspVQyuaOf+39dnMddhEDYYg+z0ce82ta/n8xPBWCp60nDEDayNjOsRgzDJKSujNfngjQTL1x6qKJj8BW/P5lLJE1nbMm9BQD9G7glJk86qh1I/tJCnij6On0m6KcdzVz8cU3sBgNeB433kGjJtpxXXmJB6Vuu5IverhyfpiB4hP9WlKa/LSzW+ZIdvl/";
@Test
public void convertkey() throws Exception {
byte[] decoded = java.util.Base64.getDecoder().decode(key1);
try {
ByteBuffer byteBuffer = ByteBuffer.wrap(decoded);
AtomicInteger position = new AtomicInteger();
//first read algorithm, should be ssh-rsa
String algorithm = readString(byteBuffer, position);
System.out.println(algorithm);
assert "ssh-rsa".equals(algorithm);
// than read exponent
BigInteger publicExponent = readMpint(byteBuffer, position);
System.out.println("publicExponent = " + publicExponent);
// than read modulus
BigInteger modulus = readMpint(byteBuffer, position);
System.out.println("modulus = " + modulus);
RSAPublicKeySpec keySpec = new RSAPublicKeySpec(modulus, publicExponent);
KeyFactory kf = KeyFactory.getInstance("RSA");
PublicKey publicKey = kf.generatePublic(keySpec);
System.out.printf("Modulus: %X%n", modulus);
System.out.printf("Public exponent: %d ... 17? Why?%n", publicExponent);
System.out.printf("See, Java class result: %s, is RSAPublicKey: %b%n", publicKey.getClass().getName(), publicKey instanceof RSAPublicKey);
byte[] pubBytes = publicKey.getEncoded();
SubjectPublicKeyInfo spkInfo = SubjectPublicKeyInfo.getInstance(pubBytes);
ASN1Primitive primitive = spkInfo.parsePublicKey();
writePEM("pubPkcs1.pem", "RSA PUBLIC KEY", primitive.getEncoded() );
} catch (Exception e) {
e.printStackTrace();
}
}
private BigInteger readMpint(ByteBuffer buffer, AtomicInteger pos){
byte[] bytes = readBytes(buffer, pos);
if(bytes.length == 0){
return BigInteger.ZERO;
}
return new BigInteger(bytes);
}
private String readString(ByteBuffer buffer, AtomicInteger pos){
byte[] bytes = readBytes(buffer, pos);
if(bytes.length == 0){
return "";
}
return new String(bytes, StandardCharsets.US_ASCII);
}
private byte[] readBytes(ByteBuffer buffer, AtomicInteger pos){
int len = buffer.getInt(pos.get());
byte buff[] = new byte[len];
for(int i = 0; i < len; i++) {
buff[i] = buffer.get(i + pos.get() + SIZEOF_INT);
}
pos.set(pos.get() + SIZEOF_INT + len);
return buff;
}
private void writePEM(String fileName, String header, byte[] content) throws IOException{
File f = new File(fileName);
FileWriter fw = new FileWriter(f);
PemObject pemObject = new PemObject(header, content);
PemWriter pemWriter = new PemWriter(fw);
pemWriter.writeObject(pemObject);
pemWriter.close();
}
output
ssh-rsa
publicExponent = 65537
modulus = 20830840075214895520187085209140532093913000825284169131015003256319574044057453999265862514662442478787476545768050679936827456316397871442660366713280574836948529205417590573371022714192162142283026905055750951413644811120376414428742776306570652231542058994893537065648692533801879404176161589597321580633083481631733302249100913607754671673449460161582195402470090670605789220193388312197339358656479303453367386441996440265509774568364904443092245613429875362588897307051303623648237031289817493491475305022256278911859470009225208699382584578527080735302630318546541433464398236590978005758010444925861059033471
Modulus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
Public exponent: 65537 ... 17? Why?
See, Java class result: sun.security.rsa.RSAPublicKeyImpl, is RSAPublicKey: true
Second part - save java object as pkcs1 public key.
If you write pubkeys.getEncoded() you got pkcs8 public key, RSA public key is different. Format described in RFC 3447 appendix A.1.1
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.
This question ans especially Ian Boyd's answer gives many details.
Bouncy castle already has class SubjectPublicKeyInfo to handle this situation.
Code to convert to pkcs8 and pkcs1 format:
byte[] pubBytes = publicKey.getEncoded();
//writePEM("pubPkcs8.pem", "PUBLIC KEY", pubBytes ); // if you need pkcs8
SubjectPublicKeyInfo spkInfo = SubjectPublicKeyInfo.getInstance(pubBytes);
ASN1Primitive primitive = spkInfo.parsePublicKey();
writePEM("pubPkcs1.pem", "RSA PUBLIC KEY", primitive.getEncoded() );
content of pubPkcs1.pem
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEApQMU+Uv1rl9ODlwbfUefVnC+OTpmlNzOVNOt5X8o2u6L6mlUe/9p
B7Sxfy6AkAl0mTJ8vEUwhg46Fw+xv7RrMhxQxwiP/2NVQyxNOgJQ865buvQ0GPdf
uoJIRox90herrOYV5Yoa20hS3wJUnZBLPIMEq7KVUMrmjn/t/XZzHXYRA2GIPs9H
HvNrWv5/MTwVgqetJwxA2sjYzrEYMwySkrozX54I0Ey9ceqiiY/AVvz+ZSyRNZ2z
JvQUA/Ru4JSZPOqodSP7SQp4o+jp9JuinHc1c/HFN7AYDXgeN95BoybacV15iQel
bruSL3q4cn6YgeIT/VpSmvy0s1vmSHb5fwIDAQAB
-----END RSA PUBLIC KEY-----
checking results with openssl
$ openssl asn1parse -in pubPkcs1.pem
0:d=0 hl=4 l= 266 cons: SEQUENCE
4:d=1 hl=4 l= 257 prim: INTEGER :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
265:d=1 hl=2 l= 3 prim: INTEGER :010001
