Crash when logging in?

I got a crash while logging into Minecraft. Here's the report that was shown in the launcher:

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 7/3/13 8:32 AM
Description: Rendering screen

java.lang.SecurityException: SHA1 digest error for b.class
	at sun.security.util.ManifestEntryVerifier.verify(Unknown Source)
	at java.util.jar.JarVerifier.processEntry(Unknown Source)
	at java.util.jar.JarVerifier.update(Unknown Source)
	at java.util.jar.JarVerifier$VerifierStream.read(Unknown Source)
	at sun.misc.Resource.getBytes(Unknown Source)
	at java.net.URLClassLoader.defineClass(Unknown Source)
	at java.net.URLClassLoader.access$100(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at bib.a(SourceFile:76)
	at bib.a(SourceFile:37)
	at avf.d(SourceFile:444)
	at avf.a(SourceFile:339)
	at avx.a(SourceFile:274)
	at ave.a(SourceFile:263)
	at bet.b(SourceFile:718)
	at atn.S(SourceFile:685)
	at atn.d(SourceFile:618)
	at net.minecraft.client.main.Main.main(SourceFile:101)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at sun.security.util.ManifestEntryVerifier.verify(Unknown Source)
	at java.util.jar.JarVerifier.processEntry(Unknown Source)
	at java.util.jar.JarVerifier.update(Unknown Source)
	at java.util.jar.JarVerifier$VerifierStream.read(Unknown Source)
	at sun.misc.Resource.getBytes(Unknown Source)
	at java.net.URLClassLoader.defineClass(Unknown Source)
	at java.net.URLClassLoader.access$100(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at bib.a(SourceFile:76)
	at bib.a(SourceFile:37)
	at avf.d(SourceFile:444)
	at avf.a(SourceFile:339)
	at avx.a(SourceFile:274)
	at ave.a(SourceFile:263)

-- Screen render details --
Details:
	Screen name: ave
	Mouse location: Scaled: (275, 138). Absolute: (550, 203)
	Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2
Stacktrace:
	at bet.b(SourceFile:718)
	at atn.S(SourceFile:685)
	at atn.d(SourceFile:618)
	at net.minecraft.client.main.Main.main(SourceFile:101)

-- System Details --
Details:
	Minecraft Version: 1.6.1
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 35580456 bytes (33 MB) / 69652480 bytes (66 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Launched Version: 1.6.1
	LWJGL: 2.9.0
	OpenGL: ATI Radeon HD 4600 Series GL version 3.3.11251 Compatibility Profile Context, ATI Technologies Inc.
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null