Lua

Understanding a few ECDSA Public Key Formats

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Describes a few ECDSA public key formats. Lua

local chilkat = 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:

local sbPem = chilkat.StringBuilder.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")

local pubKey1 = chilkat.PublicKey.new()
local success = pubKey1:load_from_string(sbPem:get_as_string())
if not success then
    print(pubKey1.last_error_text)
    return
end

-- The X9.63 format can be loaded like this:
local sbHex = chilkat.StringBuilder.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.
local numReplaced = sbHex:replace(":", "")

-- We'll need to convert hex to base64..
local bdKey = chilkat.BinData.new()
bdKey:append_encoded(sbHex:get_as_string(), "hex")

local pubKey2 = chilkat.PublicKey.new()
success = pubKey2:load_from_string(bdKey:get_encoded("base64"))
if not success then
    print(pubKey2.last_error_text)
    return
end

-- Let's get the key in pubKey2 as PEM.
-- It should be idential to the PEM above.
print(pubKey2:get_pem(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-----