[1/2] Add a protocol handler for AES CBC decryption with PKCS7 padding

Message ID 1302595694-91663-1-git-send-email-martin@martin.st
State Superseded
Headers show

Commit Message

Martin Storsjö April 12, 2011, 8:08 a.m.
This can later be extended to support other AES bit sizes,
encryption, other crypto algorithms, reading the key from a URL, etc.

In order to use it, the key and initialization vector has to be
passed via AVOptions. Since such options can't be passed to
protocols from the command line, the protocol is currently
only for libavformat internal use.
---
 libavformat/Makefile     |    1 +
 libavformat/allformats.c |    1 +
 libavformat/crypto.c     |  170 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 172 insertions(+), 0 deletions(-)
 create mode 100644 libavformat/crypto.c

Comments

Diego Biurrun April 12, 2011, 9:42 a.m. | #1
On Tue, Apr 12, 2011 at 11:08:13AM +0300, Martin Storsjö wrote:
> This can later be extended to support other AES bit sizes,
> encryption, other crypto algorithms, reading the key from a URL, etc.
> 
> --- /dev/null
> +++ b/libavformat/crypto.c
> @@ -0,0 +1,170 @@
> +/*
> + * Decryption protocol handler
> + * Copyright (c) 2011 Martin Storsjo

You skip the umlaut in your own name?

> +#define OFFSET(x) offsetof(CryptoContext, x)
> +static const AVOption options[] = {
> +{"key", "", OFFSET(key), FF_OPT_TYPE_BINARY },
> +{"iv", "", OFFSET(iv), FF_OPT_TYPE_BINARY },
> +{NULL}
> +};
> +static const AVClass crypto_class = {
> +    "crypto", av_default_item_name, options, LIBAVUTIL_VERSION_INT
> +};

That looks quite unreadable unindented and without separating empty
lines.

> +    if (av_strstart(uri, "crypto+", &nested_url)) {
> +    } else if (av_strstart(uri, "crypto:", &nested_url)) {
> +    } else {
> +        av_log(NULL, AV_LOG_ERROR, "Unsupported url %s\n", uri);
> +        ret = AVERROR(EINVAL);
> +        goto err;
> +    }

Two empty if blocks?

> +        c->outptr += size;
> +        c->outdata -= size;

extra good alignment karma awaits here

> +    c->outdata = BLOCKSIZE * blocks;
> +    c->outptr = c->outbuffer;
> +    c->indata_used += BLOCKSIZE * blocks;

ditto

> +        c->indata -= c->indata_used;
> +        c->indata_used = 0;

ditto

> +static int crypto_close(URLContext *h)
> +{
> +    CryptoContext *c = h->priv_data;
> +    if (c->hd)
> +        ffurl_close(c->hd);

unrelated: Maybe ffurl_close should be made NULL-tolerant.

Diego
Martin Storsjö April 12, 2011, 9:51 a.m. | #2
On Tue, 12 Apr 2011, Diego Biurrun wrote:

> On Tue, Apr 12, 2011 at 11:08:13AM +0300, Martin Storsjö wrote:
> > This can later be extended to support other AES bit sizes,
> > encryption, other crypto algorithms, reading the key from a URL, etc.
> > 
> > --- /dev/null
> > +++ b/libavformat/crypto.c
> > @@ -0,0 +1,170 @@
> > +/*
> > + * Decryption protocol handler
> > + * Copyright (c) 2011 Martin Storsjo
> 
> You skip the umlaut in your own name?

I normally do that within files, yes. Since Janne used the full umlaut 
version in the git author names during the git conversion, I've kept on 
using it there, though, for consistency.

> > +#define OFFSET(x) offsetof(CryptoContext, x)
> > +static const AVOption options[] = {
> > +{"key", "", OFFSET(key), FF_OPT_TYPE_BINARY },
> > +{"iv", "", OFFSET(iv), FF_OPT_TYPE_BINARY },
> > +{NULL}
> > +};
> > +static const AVClass crypto_class = {
> > +    "crypto", av_default_item_name, options, LIBAVUTIL_VERSION_INT
> > +};
> 
> That looks quite unreadable unindented and without separating empty
> lines.

Cleaned up locally. Normally, these AVOption lines get very very long, and 
skipping the indentation is a good compromise, but here it sure can be 
cleaned up.

> > +    if (av_strstart(uri, "crypto+", &nested_url)) {
> > +    } else if (av_strstart(uri, "crypto:", &nested_url)) {
> > +    } else {
> > +        av_log(NULL, AV_LOG_ERROR, "Unsupported url %s\n", uri);
> > +        ret = AVERROR(EINVAL);
> > +        goto err;
> > +    }
> 
> Two empty if blocks?

They indicate the two cases where the url was successfully parsed. 
Simplified into if (!av_strstart() && !av_strstart()) { error; }.

> > +        c->outptr += size;
> > +        c->outdata -= size;
> 
> extra good alignment karma awaits here

Wooh, extra karma for me \o/

> > +    c->outdata = BLOCKSIZE * blocks;
> > +    c->outptr = c->outbuffer;
> > +    c->indata_used += BLOCKSIZE * blocks;
> 
> ditto

Done

> > +        c->indata -= c->indata_used;
> > +        c->indata_used = 0;
> 
> ditto

Done

// Martin

Patch

diff --git a/libavformat/Makefile b/libavformat/Makefile
index 719783c..7bb479a 100644
--- a/libavformat/Makefile
+++ b/libavformat/Makefile
@@ -312,6 +312,7 @@  OBJS+= avio.o aviobuf.o
 
 OBJS-$(CONFIG_APPLEHTTP_PROTOCOL)        += applehttpproto.o
 OBJS-$(CONFIG_CONCAT_PROTOCOL)           += concat.o
+OBJS-$(CONFIG_CRYPTO_PROTOCOL)           += crypto.o
 OBJS-$(CONFIG_FILE_PROTOCOL)             += file.o
 OBJS-$(CONFIG_GOPHER_PROTOCOL)           += gopher.o
 OBJS-$(CONFIG_HTTP_PROTOCOL)             += http.o httpauth.o
diff --git a/libavformat/allformats.c b/libavformat/allformats.c
index 9398d34..6b49d21 100644
--- a/libavformat/allformats.c
+++ b/libavformat/allformats.c
@@ -234,6 +234,7 @@  void av_register_all(void)
     /* protocols */
     REGISTER_PROTOCOL (APPLEHTTP, applehttp);
     REGISTER_PROTOCOL (CONCAT, concat);
+    REGISTER_PROTOCOL (CRYPTO, crypto);
     REGISTER_PROTOCOL (FILE, file);
     REGISTER_PROTOCOL (GOPHER, gopher);
     REGISTER_PROTOCOL (HTTP, http);
diff --git a/libavformat/crypto.c b/libavformat/crypto.c
new file mode 100644
index 0000000..83461cb
--- /dev/null
+++ b/libavformat/crypto.c
@@ -0,0 +1,170 @@ 
+/*
+ * Decryption protocol handler
+ * Copyright (c) 2011 Martin Storsjo
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "avformat.h"
+#include "libavutil/aes.h"
+#include "libavutil/avstring.h"
+#include "libavutil/opt.h"
+#include "internal.h"
+#include "url.h"
+
+#define MAX_BUFFER_BLOCKS 150
+#define BLOCKSIZE 16
+
+typedef struct {
+    const AVClass *class;
+    URLContext *hd;
+    uint8_t inbuffer [BLOCKSIZE*MAX_BUFFER_BLOCKS],
+            outbuffer[BLOCKSIZE*MAX_BUFFER_BLOCKS];
+    uint8_t *outptr;
+    int indata, indata_used, outdata;
+    int eof;
+    uint8_t *key;
+    int keylen;
+    uint8_t *iv;
+    int ivlen;
+    struct AVAES *aes;
+} CryptoContext;
+
+#define OFFSET(x) offsetof(CryptoContext, x)
+static const AVOption options[] = {
+{"key", "", OFFSET(key), FF_OPT_TYPE_BINARY },
+{"iv", "", OFFSET(iv), FF_OPT_TYPE_BINARY },
+{NULL}
+};
+static const AVClass crypto_class = {
+    "crypto", av_default_item_name, options, LIBAVUTIL_VERSION_INT
+};
+
+static int crypto_open(URLContext *h, const char *uri, int flags)
+{
+    const char *nested_url;
+    int ret;
+    CryptoContext *c = h->priv_data;
+
+    if (av_strstart(uri, "crypto+", &nested_url)) {
+    } else if (av_strstart(uri, "crypto:", &nested_url)) {
+    } else {
+        av_log(NULL, AV_LOG_ERROR, "Unsupported url %s\n", uri);
+        ret = AVERROR(EINVAL);
+        goto err;
+    }
+
+    if (c->keylen < BLOCKSIZE || c->ivlen < BLOCKSIZE) {
+        av_log(NULL, AV_LOG_ERROR, "Key or IV not set\n");
+        ret = AVERROR(EINVAL);
+        goto err;
+    }
+    if (flags & (AVIO_WRONLY | AVIO_RDWR)) {
+        av_log(NULL, AV_LOG_ERROR, "Only decryption is supported currently\n");
+        ret = AVERROR(ENOSYS);
+        goto err;
+    }
+    if ((ret = ffurl_open(&c->hd, nested_url, AVIO_RDONLY)) < 0) {
+        av_log(NULL, AV_LOG_ERROR, "Unable to open input\n");
+        goto err;
+    }
+    c->aes = av_mallocz(av_aes_size);
+    if (!c->aes) {
+        ret = AVERROR(ENOMEM);
+        goto err;
+    }
+
+    av_aes_init(c->aes, c->key, 128, 1);
+
+    h->is_streamed = 1;
+
+    return 0;
+err:
+    av_free(c->key);
+    av_free(c->iv);
+    return ret;
+}
+
+static int crypto_read(URLContext *h, uint8_t *buf, int size)
+{
+    CryptoContext *c = h->priv_data;
+    int blocks;
+retry:
+    if (c->outdata > 0) {
+        size = FFMIN(size, c->outdata);
+        memcpy(buf, c->outptr, size);
+        c->outptr += size;
+        c->outdata -= size;
+        return size;
+    }
+    // We avoid using the last block until we've found EOF,
+    // since we'll remove PKCS7 padding at the end. So make
+    // sure we've got at least 2 blocks, so we can decrypt
+    // at least one.
+    while (c->indata - c->indata_used < 2*BLOCKSIZE) {
+        int n = ffurl_read(c->hd, c->inbuffer + c->indata,
+                           sizeof(c->inbuffer) - c->indata);
+        if (n <= 0) {
+            c->eof = 1;
+            break;
+        }
+        c->indata += n;
+    }
+    blocks = (c->indata - c->indata_used) / BLOCKSIZE;
+    if (!blocks)
+        return AVERROR_EOF;
+    if (!c->eof)
+        blocks--;
+    av_aes_crypt(c->aes, c->outbuffer, c->inbuffer + c->indata_used, blocks,
+                 c->iv, 1);
+    c->outdata = BLOCKSIZE * blocks;
+    c->outptr = c->outbuffer;
+    c->indata_used += BLOCKSIZE * blocks;
+    if (c->indata_used >= sizeof(c->inbuffer)/2) {
+        memmove(c->inbuffer, c->inbuffer + c->indata_used,
+                c->indata - c->indata_used);
+        c->indata -= c->indata_used;
+        c->indata_used = 0;
+    }
+    if (c->eof) {
+        // Remove PKCS7 padding at the end
+        int padding = c->outbuffer[c->outdata - 1];
+        c->outdata -= padding;
+    }
+    goto retry;
+}
+
+static int crypto_close(URLContext *h)
+{
+    CryptoContext *c = h->priv_data;
+    if (c->hd)
+        ffurl_close(c->hd);
+    av_free(c->aes);
+    av_free(c->key);
+    av_free(c->iv);
+    return 0;
+}
+
+URLProtocol ff_crypto_protocol = {
+    .name            = "crypto",
+    .url_open        = crypto_open,
+    .url_read        = crypto_read,
+    .url_close       = crypto_close,
+    .priv_data_size  = sizeof(CryptoContext),
+    .priv_data_class = &crypto_class,
+    .flags           = URL_PROTOCOL_FLAG_NESTED_SCHEME,
+};