(Ruby) Understanding a few ECDSA Public Key Formats

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Describes a few ECDSA public key formats.

Chilkat Ruby Downloads install from rubygems.org gem install chilkat or download... Ruby Library for Windows, MacOS, Linux, Alpine Linux

require 'chilkat'

# 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:

sbPem = Chilkat::CkStringBuilder.new()
sbPem.Append("-----BEGIN PUBLIC KEY-----\r\n")
sbPem.Append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET\r\n")
sbPem.Append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==\r\n")
sbPem.Append("-----END PUBLIC KEY-----\r\n")

pubKey1 = Chilkat::CkPublicKey.new()
success = pubKey1.LoadFromString(sbPem.getAsString())
if (success == false)
    print pubKey1.lastErrorText() + "\n";
    exit
end

# The X9.63 format can be loaded like this:
sbHex = Chilkat::CkStringBuilder.new()
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.
numReplaced = sbHex.Replace(":","")

# We'll need to convert hex to base64..
bdKey = Chilkat::CkBinData.new()
bdKey.AppendEncoded(sbHex.getAsString(),"hex")

pubKey2 = Chilkat::CkPublicKey.new()
success = pubKey2.LoadFromString(bdKey.getEncoded("base64"))
if (success == false)
    print pubKey2.lastErrorText() + "\n";
    exit
end

# Let's get the key in pubKey2 as PEM.
# It should be idential to the PEM above.
print pubKey2.getPem(true) + "\n";

# 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-----