B4X
Understanding a few ECDSA Public Key Formats

Describes a few ECDSA public key formats.
Chilkat B4X Downloads

B4X
Dim success As Boolean = False

'  Here we have the output of the following openssl command:  openssl ec -in key.pem -pubout -text

'  Private-Key: (256 bit)
'  priv:
'      0e:63:25:8a:73:3c:71:b6:c0:e7:a3:0f:94:b9:74:
'      e0:be:bd:46:18:be:40:7e:66:9e:21:99:85:0e:ed:
'      87:2d
'  pub:
'      04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
'      b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
'      39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
'      2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
'      12:b7:b8:6a:43
'  ASN1 OID: prime256v1
'  NIST CURVE: P-256
'  writing EC key
'  -----BEGIN PUBLIC KEY-----
'  MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
'  Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
'  -----END PUBLIC KEY-----

'  The public key is shown in two different formats.
'  The first is this:

'  pub:
'      04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
'      b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
'      39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
'      2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
'      12:b7:b8:6a:43

'  It is the ANSI X9.63 format.
'  65-bytes are the uncompressed public key (04 || X || Y) 

'  This is the same public key, but in PEM format

'  -----BEGIN PUBLIC KEY-----
'  MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
'  Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
'  -----END PUBLIC KEY-----

'  It contains ASN.1 that more explicitly identifies the key type.
'  
'  SEQUENCE (2 elem)
'    SEQUENCE (2 elem)
'      OBJECT IDENTIFIER 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type)
'      OBJECT IDENTIFIER 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve)
'    BIT STRING (520 bit) 000001000101110100011010010011111101100110111101010010011001111011100...

'  PEM format can be loaded into a Chilkat public key object like this:

Dim sbPem As ChilkatStringBuilder
sbPem.Initialize
sbPem.Append("-----BEGIN PUBLIC KEY-----" & Chr(13) & Chr(10))
sbPem.Append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET" & Chr(13) & Chr(10))
sbPem.Append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==" & Chr(13) & Chr(10))
sbPem.Append("-----END PUBLIC KEY-----" & Chr(13) & Chr(10))

Dim pubKey1 As ChilkatPublicKey
pubKey1.Initialize
success = pubKey1.LoadFromString(sbPem.GetAsString)
If success = False Then
    Log(pubKey1.LastErrorText)
    Return
End If


'  The X9.63 format can be loaded like this:
Dim sbHex As ChilkatStringBuilder
sbHex.Initialize
sbHex.Append("04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:")
sbHex.Append("b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:")
sbHex.Append("39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:")
sbHex.Append("2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:")
sbHex.Append("12:b7:b8:6a:43")

'  Get rid of the ":" chars.
Dim numReplaced As Int = sbHex.Replace(":", "")

'  We'll need to convert hex to base64..
Dim bdKey As ChilkatBinData
bdKey.Initialize
bdKey.AppendEncoded(sbHex.GetAsString, "hex")

Dim pubKey2 As ChilkatPublicKey
pubKey2.Initialize
success = pubKey2.LoadFromString(bdKey.GetEncoded("base64"))
If success = False Then
    Log(pubKey2.LastErrorText)
    Return
End If


'  Let's get the key in pubKey2 as PEM.
'  It should be idential to the PEM above.
Log(pubKey2.GetPem(True))

'  Here's the output.  You can see it's the same as the PEM above..

'  -----BEGIN PUBLIC KEY-----
'  MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
'  Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
'  -----END PUBLIC KEY-----