Added Linux support

Package.swift

@@ -16,3 +16,20 @@ let package = Package(
         .testTarget(name: "AWSSignerTests", dependencies: ["AWSSigner"]),
     ]
 )
+
+// switch for whether to use CAWSSDKOpenSSL to shim between OpenSSL versions
+#if os(Linux)
+let useOpenSSLShim = true
+#else
+let useOpenSSLShim = false
+#endif
+
+// AWSSDKSwiftCore target
+let awsSdkSwiftCoreTarget = package.targets.first(where: {$0.name == "AWSSigner"})
+
+// Decide on where we get our SSL support from. Linux usses NIOSSL to provide SSL. Linux also needs CAWSSDKOpenSSL to shim across different OpenSSL versions for the HMAC functions.
+if useOpenSSLShim {
+    package.targets.append(.target(name: "CAWSSigner"))
+    awsSdkSwiftCoreTarget?.dependencies.append("CAWSSigner")
+    package.dependencies.append(.package(url: "https://github.com/apple/swift-nio-ssl-support.git", from: "1.0.0"))
+}

Sources/AWSSigner/hash.swift

@@ -7,8 +7,44 @@
 
 import Foundation
 
-// Currently only works if CommonCrypto exists. Will look into doing something for Linux later
-#if canImport(CommonCrypto)
+// use CAWSSigner if available, otherwise use CommonCrypto
+// Package.swift includes CAWSSigner target if we are running on Linux
+#if canImport(CAWSSigner)
+
+import CAWSSigner
+
+public func sha256(_ string: String) -> [UInt8] {
+    let bytes = Array(string.utf8)
+    return sha256(bytes)
+}
+
+public func sha256(_ bytes: [UInt8]) -> [UInt8] {
+    var hash = [UInt8](repeating: 0, count: Int(SHA256_DIGEST_LENGTH))
+    SHA256(bytes, bytes.count, &hash)
+    return hash
+}
+
+public func sha256(_ buffer: UnsafeBufferPointer<UInt8>) -> [UInt8] {
+    var hash = [UInt8](repeating: 0, count: Int(SHA256_DIGEST_LENGTH))
+    SHA256(buffer.baseAddress, buffer.count, &hash)
+    return hash
+}
+
+func hmac(string: String, key: [UInt8]) -> [UInt8] {
+    let context = AWS_SIGNER_HMAC_CTX_new()
+    HMAC_Init_ex(context, key, Int32(key.count), EVP_sha256(), nil)
+    
+    let bytes = Array(string.utf8)
+    HMAC_Update(context, bytes, bytes.count)
+    var digest = [UInt8](repeating: 0, count: Int(EVP_MAX_MD_SIZE))
+    var length: UInt32 = 0
+    HMAC_Final(context, &digest, &length)
+    AWS_SIGNER_HMAC_CTX_free(context)
+    
+    return Array(digest[0..<Int(length)])
+}
+
+#elseif canImport(CommonCrypto)
 
 import CommonCrypto
 

Sources/CAWSSigner/include/c_aws_signer.h

@@ -0,0 +1,18 @@
+//
+//  c_awssdk_openssl.h
+//  AWSSDKSwiftCore
+//
+//  Created by Adam Fowler on 2019/08/08.
+//
+
+#ifndef C_AWSSDK_OPENSSL_H
+#define C_AWSSDK_OPENSSL_H
+
+#include <openssl/hmac.h>
+#include <openssl/md5.h>
+#include <openssl/sha.h>
+
+HMAC_CTX *AWS_SIGNER_HMAC_CTX_new();
+void AWS_SIGNER_HMAC_CTX_free(HMAC_CTX* ctx);
+
+#endif // C_AWSSDK_OPENSSL_H

Sources/CAWSSigner/shims.c

@@ -0,0 +1,34 @@
+//
+//  shims.c
+//  AWSSDKSwiftCore
+//
+//  Created by Adam Fowler on 2019/08/08.
+//
+
+// These are functions that shim over differences in different OpenSSL versions,
+// which are best handled by using the C preprocessor.
+#include "c_aws_signer.h"
+#include <string.h>
+
+HMAC_CTX *AWS_SIGNER_HMAC_CTX_new() {
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || (defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x2070000fL)
+    HMAC_CTX *ctx = OPENSSL_malloc(sizeof(HMAC_CTX));
+    if (ctx != NULL) {
+        HMAC_CTX_init(ctx);
+    }
+    return ctx;
+#else
+    return HMAC_CTX_new();
+#endif
+}
+
+void AWS_SIGNER_HMAC_CTX_free(HMAC_CTX* ctx) {
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || (defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x2070000fL)
+    if (ctx != NULL) {
+        HMAC_CTX_cleanup(ctx);
+        OPENSSL_free(ctx);
+    }
+#else
+    HMAC_CTX_free(ctx);
+#endif
+}