File uploading directly to S3 from an iOS app.

create_policy.rb

require 'base64'
require 'openssl'
require 'digest/sha1'

policy_document = '{
  "expiration": "2020-01-01T00:00:00Z",
	"conditions": [ 
	    {"bucket": "YOUR_BUCKET"}, 
	    ["starts-with", "$key", "uploads/"],
	    {"acl": "public-read"},
	    {"success_action_redirect": "http://localhost/"},
	    ["starts-with", "$Content-Type", ""],
	    ["content-length-range", 0, 31457280]
  ]
}'

aws_secret_key = 'YOUR_AWS_SECRET_KEY'

policy = Base64.encode64(policy_document).gsub("\n","")

signature = Base64.encode64(
    OpenSSL::HMAC.digest(
        OpenSSL::Digest::Digest.new('sha1'), 
        aws_secret_key, policy)
    ).gsub("\n","")

puts "Policy"
puts "======"
puts policy

puts "\n"
puts "Signature"
puts "========="
puts signature

S3Upload.mm

AFHTTPClient *client = [AFHTTPClient clientWithBaseURL:[NSURL URLWithString:@"https://YOUR_BUCKET.s3.amazonaws.com/"]];

NSDictionary *parameters = @{
    @"key": @"uploads/${filename}",
    @"AWSAccessKeyId": @"YOUR_AWS_ACCESS_KEY",
    @"acl": @"public-read", // or private
    @"success_action_redirect": @"http://localhost/",
    @"policy": @"YOUR_POLICY_DOCUMENT_BASE64_ENCODED",
    @"signature": @"YOUR_CALCULATED_SIGNATURE",
    @"Content-Type": @"image/jpeg"
};
NSURLRequest *request = [client multipartFormRequestWithMethod:@"POST" path:@"" parameters:parameters constructingBodyWithBlock:^(id<AFMultipartFormData> formData) {
    NSData *imageData = UIImageJPEGRepresentation([UIImage imageNamed:@"test.jpg"], 1);
    // The name of the field should be @"file"
    [formData appendPartWithFileData:imageData name:@"file" fileName:@"upload.jpg" mimeType:@"image/jpeg"];
}];
AFHTTPRequestOperation *operation = [client HTTPRequestOperationWithRequest:request success:^(AFHTTPRequestOperation *op, id body) {
    NSLog(@"Success");
} failure:^(AFHTTPRequestOperation *op, NSError *error) {
    NSLog(@"Failure %@", error);
}];

[client enqueueHTTPRequestOperation:operation];

Based on the HTML form documentation provided by Amazon.

You'll need to create the policy document and signature based on the parameter dictionary being sent with the request. Update the variables in the Ruby script and execute it to get those values.