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Signs & symbols
Glyph
Alchemy
minimal, Alchemy, protect
Black
512x512
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Miscellany Web icons
padlock, security, protect
371x512
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Buttons
security, protect, warning
Goldenrod
help, Protection, security
Teal
Block, protect, shield
OliveDrab
protect, shield, ok
Block, ban, Adware
Firebrick
Mixed
Basic icon element
security, Lock, protect
iOS7 optimized
Miu
locked, protect, security
security, secure, protect
UI
Very Basic. Android L (Lollipop)
private, Lock, protect
secure, protect, Key
Unique-Round-Blue
Unlock, password, Safe
DeepSkyBlue
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Desktop apps
16x16 Free Toolbar Icons
protect, Antivirus, yellow
Khaki
16x16