根据文件OpenSSL::HMAC.digest
返回示例表示为二进制字符串的身份验证代码。
如果在使用时遇到问题，可能需要OpenSSL::HMAC.hexdigest提供的十六进制编码形式
示例

key = 'key'
data = 'The quick brown fox jumps over the lazy dog'
digest = OpenSSL::Digest.new('sha256')

OpenSSL::HMAC.digest(digest, key, data)
#=> "\xF7\xBC\x83\xF40S\x84$\xB12\x98\xE6\xAAo\xB1C\xEFMY\xA1IF\x17Y\x97G\x9D\xBC-\x1A<\xD8"

OpenSSL::HMAC.hexdigest(digest, key, data)
#=> "f7bc83f430538424b13298e6aa6fb143ef4d59a14946175997479dbc2d1a3cd8"

试试这个：

hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), key, data)

def make_payment(user)
    @key= SecureRandom.hex(10)
    #puts @key
    @secret_key = @key
    puts " this is the  public key #{@secret_key}"
    @access_key= generate_key
    puts " this is the access key #{@access_key}"
    @name= @user.name
    puts "#{@name}"
    @time= Time.now.in_time_zone("Nairobi")
    puts "This is the time request sent #{@time}"
    @server_key = SecureRandom.base64
    puts "This is the server key #{@server_key}"
    @data = 'This request is being made from Learnida for users to make a payment'
    @digest = OpenSSL::Digest.new('sha256')
    uri = URI.parse("https://learnida.com")

    @hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), @secret_key, @access_key)
     puts "This is the HMAC #{@hmac}"
    req = Net::HTTP::Get.new(uri)
    req['Authorization'] = "TM-HMAC-SHA256 key=#{@access_key} ts=#{@time} sign=#{@hmac}"
    res = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) { |http| http.request(req) }
    @hmacdigest= OpenSSL::HMAC.digest(@digest, @server_key, @data)
    puts" This is the HMAC:SHA-256:   #{@hmacdigest}" 
    #puts res.body
    #=> "\xF7\xBC\x83\xF40S\x84$\xB12\x98\xE6\xAAo\xB1C\xEFMY\xA1IF\x17Y\x97G\x9D\xBC-\x1A<\xD8"
    @sslkey= OpenSSL::HMAC.hexdigest(@digest, @server_key, @data)
    puts @sslkey

在我的例子（Ticketmatic）中，我必须像上面那样创建HMAC，并向包含HMAC的请求添加Authorization头。

hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), secret_key, access_key + name + time)
req = Net::HTTP::Get.new(uri)
req['Authorization'] = "TM-HMAC-SHA256 key=#{access_key} ts=#{time} sign=#{hmac}"
res = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) { |http| http.request(req) }

你可以找到完整的要点here
还有一个解释更多here的博客