Added ssh_rsa_key_util.py.

Set of utility functions to read and write rsa keys in the openssh
format.

1 files changed, 206 insertions, 0 deletions

diff --git a/ssh_rsa_key_util.py b/ssh_rsa_key_util.py
new file mode 100644
index 0000000..11d322e
--- /dev/null
+++ b/ssh_rsa_key_util.py
@@ -0,0 +1,206 @@
+import base64
+import struct
+import filecmp
+
+# need pyasn for DER parsing and generating
+from pyasn1.type import univ
+from pyasn1.codec.der import decoder, encoder
+
+def read_int(buffer, i):
+    "Read 32bit integer from buffer."
+
+    (l,) = struct.unpack('!I', buffer[i:i + 4])
+    i += 4
+    return (l, i)
+
+
+def read_chunk(buffer, i):
+    "Read chunk from buffer."
+
+    # first grab length of chunk
+    (l, i) = read_int(buffer, i)
+    if l > 1000000:
+        # just in case... if this happens, then something is way off
+        raise Exception("got chunk length of %d, that's certainly too long" % l)
+
+    # read chunk of length l
+    #(s,) = struct.unpack('!' + '%ds' % l, buffer[i:i + l])
+    (s,) = struct.unpack('!{0:d}s'.format(l), buffer[i:i + l])
+    i += l
+    return (s, i)
+
+
+def unpack_bigint(buffer):
+    "Turn binary chunk into integer."
+
+    v = 0
+    for c in buffer:
+        v *= 256
+        v += ord(c)
+
+    return v
+
+
+def pack_bigint(v):
+    "Pack integer into binary chunk."
+
+    chunk = ''
+    rest = v
+    while rest:
+        chunk = chr(rest % 256) + chunk
+        rest //= 256
+
+    # add a zero byte if the highest bit is 1, so it won't be negative
+    if ord(chunk[0]) & 128:
+        chunk = chr(0) + chunk
+
+    return chunk
+
+
+def read_rsa_pub(filename):
+    """Read RSA public key file. Structure:
+
+       ssh-rsa base64data user@host
+
+       base64data: [7]ssh-rsa[len][e-data][len][n-data]
+    """
+
+    [prefix, data, host] = file(filename, 'r').read().split()
+    raw = base64.b64decode(data)
+
+    # read type string
+    i = 0
+    (s, i) = read_chunk(raw, i)
+    if s != 'ssh-rsa':
+        raise Exception("expected string 'ssh-rsa' but got '%s'" % s)
+
+    # grab e
+    (s, i) = read_chunk(raw, i)
+    e = unpack_bigint(s)
+
+    # grab n
+    (s, i) = read_chunk(raw, i)
+    n = unpack_bigint(s)
+
+    return (n, e, host)
+
+
+def write_rsa_pub(filename, n, e, host):
+    """Write RSA public key file. Structure:
+
+       ssh-rsa base64data user@host
+
+       base64data: [7]ssh-rsa[len][e-data][len][n-data]
+    """
+
+    e_str = pack_bigint(e)
+    n_str = pack_bigint(n)
+    # pack e and n properly into the raw data
+    raw = struct.pack('!I7sI{0:d}sI{1:d}s'.format(len(e_str), len(n_str)), 7, 'ssh-rsa',
+                                        len(e_str), e_str, len(n_str), n_str)
+    # assemble file content and save it
+    content = "ssh-rsa {0!s} {1!s}\n".format(base64.b64encode(raw), host)
+    file(filename, 'w').write(content)
+
+
+def read_rsa_pri(filename):
+    """Read RSA private key file. Structure:
+
+       -----BEGIN RSA PRIVATE KEY-----
+       base64data
+       -----END RSA PRIVATE KEY-----
+
+       base64data DER structure:
+
+       RSAPrivateKey ::= SEQUENCE {
+         version Version,
+         modulus INTEGER,         -- n
+         publicExponent INTEGER,  -- e
+         privateExponent INTEGER, -- d
+         prime1 INTEGER,          -- p
+         prime2 INTEGER,          -- q
+         exponent1 INTEGER,       -- d mod (p - 1)
+         exponent2 INTEGER,       -- d mod (q - 1)
+         coefficient INTEGER      -- q^-1 mod p
+       }
+    """
+
+    # grab only the lines between the --- * --- lines, glue them together
+    data = ''.join(filter(lambda x: x and x[0] != '-',
+                          file(filename, 'r').read().split('\n')))
+    # decode from base64
+    raw = base64.b64decode(data)
+    # parse DER structure
+    der = decoder.decode(raw)
+    (version, n, e, d, p, q, e1, e2, c) = (int(x) for x in der[0])
+
+    return (n, e, d, p, q, e1, e2, c)
+
+
+def write_rsa_pri(filename, n, e, d, p, q, e1, e2, c):
+    """Write RSA private key file. Structure:
+
+       -----BEGIN RSA PRIVATE KEY-----
+       base64data
+       -----END RSA PRIVATE KEY-----
+
+       base64data DER structure:
+
+       RSAPrivateKey ::= SEQUENCE {
+         version Version,
+         modulus INTEGER,         -- n
+         publicExponent INTEGER,  -- e
+         privateExponent INTEGER, -- d
+         prime1 INTEGER,          -- p
+         prime2 INTEGER,          -- q
+         exponent1 INTEGER,       -- d mod (p - 1)
+         exponent2 INTEGER,       -- d mod (q - 1)
+         coefficient INTEGER      -- q^-1 mod p
+       }
+    """
+
+    seq = (
+            univ.Integer(0),
+            univ.Integer(n),
+            univ.Integer(e),
+            univ.Integer(d),
+            univ.Integer(p),
+            univ.Integer(q),
+            univ.Integer(e1),
+            univ.Integer(e2),
+            univ.Integer(c),
+          )
+    struct = univ.Sequence()
+    for i in xrange(len(seq)):
+        struct.setComponentByPosition(i, seq[i])
+
+    # build DER structure
+    raw = encoder.encode(struct)
+    # encode to base64
+    data = base64.b64encode(raw)
+
+    # chop data up into lines of certain width
+    width = 64
+    chopped = [data[i:i + width] for i in xrange(0, len(data), width)]
+    # assemble file content
+    content = """-----BEGIN RSA PRIVATE KEY-----
+%s
+-----END RSA PRIVATE KEY-----
+""" % '\n'.join(chopped)
+    file(filename, 'w').write(content)
+
+if __name__ == '__main__' :
+    ssh_keys_directory='/home/guillaume/.ssh/'
+    print 'Testing public key reading...'
+    (n,e,host)=read_rsa_pub(ssh_keys_directory + 'id_rsa.pub')
+    write_rsa_pub(ssh_keys_directory + 'id_rsa_test.pub',n,e,host)
+    if filecmp.cmp(ssh_keys_directory + 'id_rsa.pub',ssh_keys_directory + 'id_rsa_test.pub'):
+        print 'test succesful'
+        
+    print 'Testing private key reading...'
+    (n, e, d, p, q, e1, e2, c)=read_rsa_pri('/home/guillaume/.ssh/id_rsa')
+    write_rsa_pri('/home/guillaume/.ssh/id_rsa_test',n, e, d, p, q, e1, e2, c)
+    if filecmp.cmp(ssh_keys_directory + 'id_rsa',ssh_keys_directory + 'id_rsa_test'):
+        print 'test succesful'
+        
+    
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